They did the same thing in North Carolina to little measurable effect:

The NAACP of Maryland is launching a radio ad on Tuesday urging support of the same-sex marriage referendum on next month’s ballot, the group said.

... African-American voters are seen by both sides as a key demographic. Opponents of same-sex marriage are working through black churches . Supporters were heartened this summer by endorsements of same-sex marriage by both President Obama and the NAACP.

In the new radio ad, Bond also argues that the issue is not primarily about religion. “I believe people of faith understand this isn’t about any one religious belief – it’s about protecting the civil right to make a lifelong commitment to the person you love,” he says. -- WaPo's Maryland Politics blog