Sure, but a lot of folks wanted to turn out, even if they weren't black themselves, to elect the first African-American President. Not to mention that while there arent huge numbers of blacks, in a close race like Prop 8, their turnout would have had an impact on Prop 8. Also, 2008 Obama was pretty damn charismatic on his own that he could inspire more Dem turnout generally nationwide. Butterfly that away, and that's a lot of OTL Yes voters not showing up.Bush also had to appease his socially conservative base. In contrast, I don't think that social conservatives made up a large part of the Dem base even back in 2004.
I don't know how crucial this was. After all, California has relatively few African-Americans.