BLACKROCK MUNICIPAL BOND FUND, INC.

BlackRock National Municipal Fund

(the “Fund”)

Class K Shares

Supplement dated May 30, 2018

to the Summary Prospectus and Prospectus, each dated October 27, 2017, as supplemented to date

Effective May 31, 2018, BlackRock Advisors, LLC (“BlackRock”) has agreed to adjust the caps on total expenses to reduce the net expenses paid by shareholders of the Fund. To achieve these expense caps, BlackRock has agreed to waive and/or reimburse fees and/or expenses if the Fund’s annual fund operating expenses, excluding certain expenses described in the prospectus, exceed a certain limit for the Fund’s Class K Shares. Accordingly, effective May 31, 2018, the Fund’s Summary Prospectus and Prospectus are amended as follows:

The section of the Fund’s Summary Prospectus entitled “Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund” and the section of the Fund’s Prospectus entitled “Fund Overview — Key Facts About BlackRock National Municipal Fund — Fees and Expenses of the Fund” are deleted in their entirety and replaced with the following:

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold Class K Shares of the Fund.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)	Class K Shares
Management Fee1,2,3		0.39	%
Distribution and/or Service 12b-1 Fees		None
Other Expenses		0.06	%
Interest Expense		0.01%
Miscellaneous Other Expenses		0.05%
Acquired Fund Fees and Expenses3		0.01	%
Total Annual Fund Operating Expenses3		0.46	%
Fee Waivers and/or Expense Reimbursements2,4		(0.06	)%
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements2,4		0.40	%

¹  The Management Fee has been restated to reflect current fees.

²  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 34, BlackRock Advisors, LLC (“BlackRock”) has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through October 31, 2019. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of BlackRock Municipal Bond Fund, Inc. (the “Corporation”) or by a vote of a majority of the outstanding voting securities of the Fund.

³  The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current fees and Acquired Fund Fees and Expenses.

⁴  As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 34, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.38% of average daily net assets through October 31, 2019. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of the Corporation or by a vote of a majority of the outstanding voting securities of the Fund.

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

	1 Year		3 Years		5 Years		10 Years
Class K Shares	$	41	$	141	$	250	$	568

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 83% of the average value of its portfolio.

The sixth paragraph in the section of the Prospectus entitled “Management of the Funds — BlackRock” is deleted in its entirety and replaced with the following:

BlackRock has contractually agreed to waive the management fee with respect to any portion of each Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through October 31, 2018 for the Short-Term Fund and through October 31, 2019 for the National Fund. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of the Corporation or by a vote of a majority of the outstanding voting securities of a Fund.

The section of the table immediately following the ninth paragraph in the section of the Prospectus entitled “Management of the Funds — BlackRock,” as it relates solely to the Fund, is deleted in its entirety and replaced with the following:

	Contractual Caps1 on Total Annual Fund Operating Expenses2 (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses)
National Fund
Class K Shares		0.38	%

¹  With respect to the National Fund, the contractual cap is in effect through October 31, 2019. With respect to the Short-Term Fund, the contractual cap is in effect through October 31, 2018. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested directors of the Corporation or by a vote of a majority of the outstanding voting securities of a Fund.

²  As a percentage of average daily net assets.

Shareholders should retain this Supplement for future reference.

PR2-MBFN-K-0518SUP

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