Exhibit 11(a)

December 11, 2008

BlackRock Municipal Bond Fund, Inc.
100 Bellevue Parkway
Wilmington, DE 19809

Ladies and Gentlemen:

You have requested us, as counsel to BlackRock Municipal Bond Fund, Inc. (the
"Corporation"), a corporation organized under the laws of the State of Maryland,
on behalf of BlackRock National Municipal Fund (the "Fund"), a series of the
Corporation, to furnish you with this opinion in connection with the
Corporation's Pre-Effective Amendment No. 1 of the Registration Statement on
Form N-14 under the Securities Act of 1933, as amended, to be filed with the
Securities and Exchange Commission on or about December 11, 2008 (the
"Registration Statement"), registering Investor A, Investor B, Investor C,
Investor C1 and Institutional shares of common stock, par value $.10 per share,
of the Fund (the "Shares"), to be issued pursuant to a Plan of Reorganization
(the "Plan") adopted by the Corporation on behalf of the Fund and by BlackRock
Multi-State Municipal Series Trust (the "Target Trust"), a business trust
organized under the laws of the Commonwealth of Massachusetts, on behalf of its
series BlackRock Florida Municipal Bond Fund (the "Target Fund"). The Plan
provides for the proposed acquisition by the Fund of all of the assets of the
Target Fund solely in exchange for the Shares and the assumption by the Fund of
certain stated liabilities of the Target Fund.

We have examined the Registration Statement, substantially in the form in which
it is to become effective, the Corporation's Articles of Incorporation and
By-Laws, resolutions adopted by the Board of Directors of the Corporation
relating to the authorization of the sale and issuance of the Shares and the
approval of the Plan (the "Resolutions") and the form of the Plan to be included
in the Proxy Statement/Prospectus included in the Registration Statement. We
have also examined such other records, documents, papers, statutes and
authorities as we have deemed necessary to form a basis for the opinion
hereinafter expressed.

In our examination of the materials described above, we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to us
as originals and the conformity to original documents of all copies submitted to
us. As to various questions of fact material to our opinion, we have relied upon
statements and certificates of officers and representatives of the Corporation
and others, which facts we have not independently verified. We have further
assumed that the Plan will be



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delivered in substantially the same form as that included in the Registration
Statement and that upon such delivery, it will constitute the legal, valid and
binding obligation of the Corporation, enforceable against the Corporation in
accordance with its terms.

Based upon the foregoing and subject to the assumptions and qualification herein
set forth, we advise you that, the Shares have been duly and validly authorized
and, when issued under the circumstances contemplated in the Registration
Statement and upon the terms set forth in the Plan, will be legally issued,
fully paid and non-assessable.

We hereby consent to the reference to us in the Registration Statement and the
filing of this opinion as an exhibit to the Registration Statement.

We are members of the Bar of the State of New York only and do not opine as to
the laws of any jurisdiction other than the laws of the State of New York and
the federal laws of the United States, and the opinions set forth above are,
accordingly, limited to the laws of those jurisdictions. As to matters governed
by the laws of the State of Maryland, we have relied upon the opinion of Miles &
Stockbridge P.C. (which is attached hereto).

Very truly yours,

/s/ Willkie Farr & Gallagher LLP