| Form N-CSR Cover | 6 Months Ended |
| Nov. 30, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | BlackRock Municipal Bond Fund, Inc. |
| Entity Central Index Key | 0000225635 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Nov. 30, 2024 |
| Shareholder Report | 6 Months Ended | |
| Nov. 30, 2024 USD ($) Holding | ||
| Shareholder Report [Line Items] | ||
| Document Type | N-CSRS | |
| Amendment Flag | false | |
| Registrant Name | BlackRock Municipal Bond Fund, Inc. | |
| Entity Central Index Key | 0000225635 | |
| Entity Investment Company Type | N-1A | |
| Document Period End Date | Nov. 30, 2024 | |
| C000234661 [Member] | ||
| Shareholder Report [Line Items] | ||
| Fund Name | BlackRock Impact Municipal Fund | |
| Class Name | Institutional Shares | |
| Trading Symbol | MPICX | |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about BlackRock Impact Municipal Fund (the “Fund”) for the period of June 1, 2024 to November 30, 2024. | |
| Shareholder Report Annual or Semi-Annual | semi-annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports | |
| Additional Information Phone Number | (800) 441‑7762 | |
| Additional Information Website | blackrock.com/fundreports | |
| Expenses [Text Block] | What were the Fund costs for the last six months ? (based on a hypothetical $10,000 investment) Class name Costs of a $10,000 Costs paid as a percentage of a Institutional Shares $ 0.58 % (a) (a) Annualized. | |
| Expenses Paid, Amount | $ 30 | |
| Expense Ratio, Percent | 0.58% | [1] |
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future results. | |
| Average Annual Return [Table Text Block] | Average annual total returns 6-Month 1 Year Since Fund Institutional Shares 4.63 % 6.37 % 3.01 % Bloomberg Municipal Bond Index 4.54 4.93 1.78 Customized Reference Benchmark 5.18 7.14 2.18 | |
| Performance Inception Date | Mar. 14, 2022 | |
| No Deduction of Taxes [Text Block] | Performance results do not reflect the deduction of taxes that a shareholder would pay on fund distributions or on the redemption or sale of fund shares. | |
| Updated Performance Information Location [Text Block] | Visit blackrock.com | |
| Net Assets | $ 50,482,762 | |
| Holdings Count | Holding | 82 | |
| Investment Company Portfolio Turnover | 2% | |
| Additional Fund Statistics [Text Block] | Key Fund statistics Net Assets $ Number of Portfolio Holdings 82 Portfolio Turnover Rate 2 % | |
| Holdings [Text Block] | What did the Fund invest in? (as of November 30, 2024) Sector allocation Sector (a) Percent of Total (b) Education 17.7 % Housing 17.3 % Transportation 13.8 % Health 12.6 % County/City/Special District/School District 11.5 % Utilities 9.9 % State 9.9 % Corporate 7.3 % Credit quality allocation Credit Rating (c) Percent of Total (b) AAA/Aaa 6.1 % AA/Aa 54.5 % A 20.3 % BBB/Baa 7.5 % BB/Ba 3.9 % N/R 7.7 % (a) For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes. (b) Excludes short-term securities. (c) For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. | |
| Credit Quality Explanation [Text Block] | For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. | |
| Credit Ratings Selection [Text Block] | For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. | |
| C000234660 [Member] | ||
| Shareholder Report [Line Items] | ||
| Fund Name | BlackRock Impact Municipal Fund | |
| Class Name | Investor A Shares | |
| Trading Symbol | MPDCX | |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about BlackRock Impact Municipal Fund (the “Fund”) for the period of June 1, 2024 to November 30, 2024. | |
| Shareholder Report Annual or Semi-Annual | semi-annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports | |
| Additional Information Phone Number | (800) 441‑7762 | |
| Additional Information Website | blackrock.com/fundreports | |
| Expenses [Text Block] | What were the Fund costs for the last six months ? (based on a hypothetical $10,000 investment) Class name Costs of a $10,000 Costs paid as a percentage of a Investor A Shares $ 0.83 % (a) (a) Annualized. | |
| Expenses Paid, Amount | $ 43 | |
| Expense Ratio, Percent | 0.83% | [2] |
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future results. | |
| Average Annual Return [Table Text Block] | Average annual total returns 6-Month 1 Year Since Fund Investor A Shares 4.39 % 6.01 % 2.77 % Investor A Shares (with sales charge) (0.05 ) 1.50 1.14 Bloomberg Municipal Bond Index 4.54 4.93 1.78 Customized Reference Benchmark 5.18 7.14 2.18 | |
| Performance Inception Date | Mar. 14, 2022 | |
| No Deduction of Taxes [Text Block] | Performance results do not reflect the deduction of taxes that a shareholder would pay on fund distributions or on the redemption or sale of fund shares. | |
| Updated Performance Information Location [Text Block] | Visit blackrock.com | |
| Net Assets | $ 50,482,762 | |
| Holdings Count | Holding | 82 | |
| Investment Company Portfolio Turnover | 2% | |
| Additional Fund Statistics [Text Block] | Key Fund statistics Net Assets $ Number of Portfolio Holdings 82 Portfolio Turnover Rate 2 % | |
| Holdings [Text Block] | What did the Fund invest in? (as of November 30, 2024) Sector allocation Sector (a) Percent of Total (b) Education 17.7 % Housing 17.3 % Transportation 13.8 % Health 12.6 % County/City/Special District/School District 11.5 % Utilities 9.9 % State 9.9 % Corporate 7.3 % Credit quality allocation Credit Rating (c) Percent of Total (b) AAA/Aaa 6.1 % AA/Aa 54.5 % A 20.3 % BBB/Baa 7.5 % BB/Ba 3.9 % N/R 7.7 % (a) For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes. (b) Excludes short-term securities. (c) For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. | |
| Credit Quality Explanation [Text Block] | For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. | |
| Credit Ratings Selection [Text Block] | For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. | |
| C000234662 [Member] | ||
| Shareholder Report [Line Items] | ||
| Fund Name | BlackRock Impact Municipal Fund | |
| Class Name | Class K Shares | |
| Trading Symbol | MPKCX | |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about BlackRock Impact Municipal Fund (the “Fund”) for the period of June 1, 2024 to November 30, 2024. | |
| Shareholder Report Annual or Semi-Annual | semi-annual shareholder report | |
| Additional Information [Text Block] | You can find additional information about the Fund at blackrock.com/fundreports | |
| Additional Information Phone Number | (800) 441‑7762 | |
| Additional Information Website | blackrock.com/fundreports | |
| Expenses [Text Block] | What were the Fund costs for the last six months ? (based on a hypothetical $10,000 investment) Class name Costs of a $10,000 Costs paid as a percentage of a Class K Shares $ 0.53 % (a) (a) Annualized. | |
| Expenses Paid, Amount | $ 27 | |
| Expense Ratio, Percent | 0.53% | [3] |
| Performance Past Does Not Indicate Future [Text] | Past performance is not an indication of future results. | |
| Average Annual Return [Table Text Block] | Average annual total returns 6-Month 1 Year Since Fund Class K Shares 4.55 % 6.32 % 3.06 % Bloomberg Municipal Bond Index 4.54 4.93 1.78 Customized Reference Benchmark 5.18 7.14 2.18 | |
| Performance Inception Date | Mar. 14, 2022 | |
| No Deduction of Taxes [Text Block] | Performance results do not reflect the deduction of taxes that a shareholder would pay on fund distributions or on the redemption or sale of fund shares. | |
| Updated Performance Information Location [Text Block] | Visit blackrock.com | |
| Net Assets | $ 50,482,762 | |
| Holdings Count | Holding | 82 | |
| Investment Company Portfolio Turnover | 2% | |
| Additional Fund Statistics [Text Block] | Key Fund statistics Net Assets $ Number of Portfolio Holdings 82 Portfolio Turnover Rate 2 % | |
| Holdings [Text Block] | What did the Fund invest in? (as of November 30, 2024) Sector allocation Sector (a) Percent of Total (b) Education 17.7 % Housing 17.3 % Transportation 13.8 % Health 12.6 % County/City/Special District/School District 11.5 % Utilities 9.9 % State 9.9 % Corporate 7.3 % Credit quality allocation Credit Rating (c) Percent of Total (b) AAA/Aaa 6.1 % AA/Aa 54.5 % A 20.3 % BBB/Baa 7.5 % BB/Ba 3.9 % N/R 7.7 % (a) For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes. (b) Excludes short-term securities. (c) For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. | |
| Credit Quality Explanation [Text Block] | For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change. | |
| Credit Ratings Selection [Text Block] | For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. | |
| [1]Annualized.[2]Annualized.[3]Annualized. | ||
| Shareholder Report, Line Graph (Details) | 6 Months Ended | 12 Months Ended | 33 Months Ended |
| Nov. 30, 2024 | Nov. 30, 2024 | Nov. 30, 2024 | |
| C000234661 [Member] | |||
| Account Value [Line Items] | |||
| Line Graph and Table Measure Name | Institutional Shares | ||
| Average Annual Return, Percent | 4.63% | 6.37% | 3.01% |
| C000234660 [Member] | |||
| Account Value [Line Items] | |||
| Line Graph and Table Measure Name | Investor A Shares (with sales charge) | ||
| Average Annual Return, Percent | (0.05%) | 1.50% | 1.14% |
| C000234660 [Member] | Without Sales Load [Member] | |||
| Account Value [Line Items] | |||
| Line Graph and Table Measure Name | Investor A Shares | ||
| Average Annual Return, Percent | 4.39% | 6.01% | 2.77% |
| C000234662 [Member] | |||
| Account Value [Line Items] | |||
| Line Graph and Table Measure Name | Class K Shares | ||
| Average Annual Return, Percent | 4.55% | 6.32% | 3.06% |
| Bloomberg Municipal Bond Index [Member] | |||
| Account Value [Line Items] | |||
| Line Graph and Table Measure Name | Bloomberg Municipal Bond Index | ||
| Average Annual Return, Percent | 4.54% | 4.93% | 1.78% |
| Customized Reference Benchmark [Member] | |||
| Account Value [Line Items] | |||
| Line Graph and Table Measure Name | Customized Reference Benchmark | ||
| Average Annual Return, Percent | 5.18% | 7.14% | 2.18% |
| Shareholder Report, Holdings (Details) | Nov. 30, 2024 | |
| C000234661 [Member] | N R [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 7.70% | [1],[2] |
| C000234661 [Member] | Standard & Poor's, AAA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 6.10% | [1],[2] |
| C000234661 [Member] | Standard & Poor's, AA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 54.50% | [1],[2] |
| C000234661 [Member] | Standard & Poor's, A Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 20.30% | [1],[2] |
| C000234661 [Member] | Standard & Poor's, BBB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 7.50% | [1],[2] |
| C000234661 [Member] | Standard & Poor's, BB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 3.90% | [1],[2] |
| C000234661 [Member] | Healthcare Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 12.60% | [1],[3] |
| C000234661 [Member] | Transportation Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 13.80% | [1],[3] |
| C000234661 [Member] | Utilities Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9.90% | [1],[3] |
| C000234661 [Member] | Education [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 17.70% | [1],[3] |
| C000234661 [Member] | Housing [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 17.30% | [1],[3] |
| C000234661 [Member] | County City Special District School District [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 11.50% | [1],[3] |
| C000234661 [Member] | State [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9.90% | [1],[3] |
| C000234661 [Member] | Corporate Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 7.30% | [1],[3] |
| C000234660 [Member] | N R [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 7.70% | [4],[5] |
| C000234660 [Member] | Standard & Poor's, AAA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 6.10% | [4],[5] |
| C000234660 [Member] | Standard & Poor's, AA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 54.50% | [4],[5] |
| C000234660 [Member] | Standard & Poor's, A Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 20.30% | [4],[5] |
| C000234660 [Member] | Standard & Poor's, BBB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 7.50% | [4],[5] |
| C000234660 [Member] | Standard & Poor's, BB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 3.90% | [4],[5] |
| C000234660 [Member] | Healthcare Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 12.60% | [4],[6] |
| C000234660 [Member] | Transportation Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 13.80% | [4],[6] |
| C000234660 [Member] | Utilities Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9.90% | [4],[6] |
| C000234660 [Member] | Education [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 17.70% | [4],[6] |
| C000234660 [Member] | Housing [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 17.30% | [4],[6] |
| C000234660 [Member] | County City Special District School District [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 11.50% | [4],[6] |
| C000234660 [Member] | State [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9.90% | [4],[6] |
| C000234660 [Member] | Corporate Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 7.30% | [4],[6] |
| C000234662 [Member] | N R [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 7.70% | [7],[8] |
| C000234662 [Member] | Standard & Poor's, AAA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 6.10% | [7],[8] |
| C000234662 [Member] | Standard & Poor's, AA Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 54.50% | [7],[8] |
| C000234662 [Member] | Standard & Poor's, A Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 20.30% | [7],[8] |
| C000234662 [Member] | Standard & Poor's, BBB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 7.50% | [7],[8] |
| C000234662 [Member] | Standard & Poor's, BB Rating [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 3.90% | [7],[8] |
| C000234662 [Member] | Healthcare Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 12.60% | [7],[9] |
| C000234662 [Member] | Transportation Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 13.80% | [7],[9] |
| C000234662 [Member] | Utilities Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9.90% | [7],[9] |
| C000234662 [Member] | Education [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 17.70% | [7],[9] |
| C000234662 [Member] | Housing [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 17.30% | [7],[9] |
| C000234662 [Member] | County City Special District School District [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 11.50% | [7],[9] |
| C000234662 [Member] | State [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 9.90% | [7],[9] |
| C000234662 [Member] | Corporate Sector [Member] | ||
| Holdings [Line Items] | ||
| Percent of Total Investments | 7.30% | [7],[9] |
| [1]Excludes short-term securities.[2]For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change.[3]For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes.[4]Excludes short-term securities.[5]For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change.[6]For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes.[7]Excludes short-term securities.[8]For purposes of this report, credit quality ratings shown above reflect the highest rating assigned by either S&P Global Ratings or Moody’s Investors Service, Inc. if ratings differ. These rating agencies are independent, nationally recognized statistical rating organizations and are widely used. Investment grade ratings are credit ratings of BBB/Baa or higher. Below investment grade ratings are credit ratings of BB/Ba or lower. Investments designated N/R are not rated by either rating agency. Unrated investments do not necessarily indicate low credit quality. Credit quality ratings are subject to change.[9]For purposes of this report, sector sub-classifications may differ from those utilized by the Fund for compliance purposes. | ||
