Many newspapers are reporting that black support of the California gay marriage ban was at a 70% level (e.g., Detroit Free Press 70% of demographic backed initiative). Based on analysis of a similar Ohio measure in 2004, I think that this figure, based on a CNN exit poll, is too high. It's fairly easy to get an accurate reading of black support of any ballot measure simply by looking at the precinct-level results. Since American cities are still largely segregated, there are many precincts that are nearly 100% of one race or another. Since blacks tend to vote Democratic at 90%+ levels, black support for the ballot initiative can be gauged merely by inspection.
I've been waiting for a major California city to post precinct-level results, but haven't seen any thus far. In lieu of that for the time being, and with the expectation that the California results will match those elsewhere, here is a scatterplot of the 765 Miami-Dade precincts, with Obama support plotted against Florida Issue 2 support:
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It can be seen that those precincts voting at 90%+ levels for Obama, surely largely black precincts, supported Issue 2 at much less than 70%. In fact, the 118 precincts with Obama support over 90% averaged 55% support for Issue 2.
For comaprison, here's my 2004 analysis of the Ohio results: Issue 1: Wedge Issue in the Ohio 2004 Election. In that case, Columbus black precincts voted for the ban at about 55%; Cleveland black precincts were a little higher at about 60%.
Clearly, while black support of gay marriage bans doesn't approach 70%, it is higher than in other Democratic-leaning groupings. In broad strokes, plots such as above come out in, ironically, a wedge shape, with conservative Republican fundies united against gay marriage at high levels on the left of the chart, and blacks and white liberals diverging on the right:
An interesting analysis, although I haven't done it for these Miami-Dade precincts, is to see where specific neighborhoods fall on the scatterplot. But based on previous results, the "white liberal" region will include university precincts, "young urban sophisticate" areas and of course concentrations of gay residents, where the marriage ban has low support and Democratic percentage is relatively high, ~70%.
The upper left, "white Republican" area will include nearly 100% white middle-class exurbs. Interestingly, in the cases of Cleveland and Columbus in 2004, wealthy Republican areas voted against the ban (e.g., Chagrin Falls, Upper Arlington). So it's basically hick Republicans who vote for the ban at the highest levels, while wealthy old-school Republicans are a bit more socially liberal, and in fact many of them may be gay or have gay friends.
Again, precinct-level data has yet to come out for Los Angeles, San Francisco, Sacramento, etc., but when they do I'd be willing to bet anyone a venti latte that the results will be similar to the above.