BLACKROCK MUNICIPAL BOND FUND, INC.

BlackRock Impact Municipal Fund

(the “Fund”)

Supplement dated March 2, 2023 (the “Supplement”) to the Fund’s

Summary Prospectuses and Prospectuses, each dated September 28, 2022

The following changes are made to the Fund’s Summary Prospectuses and Prospectuses, as applicable:

The section of the Summary Prospectuses entitled “Key Facts About BlackRock Impact Municipal Fund — Portfolio Managers” and the section of the Prospectuses entitled “Fund Overview — Key Facts About BlackRock Impact Municipal Fund — Portfolio Managers” are deleted in their entirety and replaced with the following:

Portfolio Managers

Name	Portfolio Manager of the Fund Since	Title
Michael Kalinoski, CFA	2022	Director of BlackRock, Inc.
Kristi Manidis	2022	Director of BlackRock, Inc.

The section of the Prospectuses entitled “Details About the Fund — How the Fund Invests — About the Portfolio Management Team of the Fund” is deleted in its entirety and replaced with the following:

ABOUT THE PORTFOLIO MANAGEMENT TEAM OF THE FUND

The Fund is managed by a team of financial professionals. Michael Kalinoski, CFA, and Kristi Manidis are the portfolio managers and are jointly and primarily responsible for the day-to-day management of the Fund. Please see “Management of the Fund — Portfolio Manager Information” for additional information about the portfolio management team.

The section of the Prospectuses entitled “Management of the Fund — Portfolio Manager Information” is deleted in its entirety and replaced with the following:

Portfolio Manager Information

Information regarding the portfolio managers of the Fund is set forth below. Further information regarding the portfolio managers, including other accounts managed, compensation, ownership of Fund shares, and possible conflicts of interest, is available in the Fund’s SAI.

The Fund is managed by a team of financial professionals. Michael Kalinoski, CFA, and Kristi Manidis are jointly and primarily responsible for the day-to-day management of the Fund.

Portfolio Manager	Primary Role	Since	Title and Recent Biography
Michael Kalinoski, CFA	Jointly and primarily responsible for the day-to-day management of the Fund’s portfolio, including setting the Fund’s overall investment strategy and overseeing the management of the Fund.	2022	Director of BlackRock, Inc. since 2006.
Kristi Manidis	Jointly and primarily responsible for the day-to-day management of the Fund’s portfolio, including setting the Fund’s overall investment strategy and overseeing the management of the Fund.	2022	Director of BlackRock, Inc. since 2016; Vice President of BlackRock, Inc. from 2011 to 2015; Associate of BlackRock, Inc. from 2006 to 2010.

Shareholders should retain this Supplement for future reference.

PRO-IMPF-0323SUP

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