According to the 2010 Census, only 6% of SF residents are black. That’s less than half of the national percentage.
Most of the blacks in the Bay Area live closer to Oakland and Richmond up towards Vallejo.
I think the other slant to the issue is that only 38-percent of the residents were born there in California. It’s a city made up of people who moved there, and aren’t part of the traditional San Francisco culture. Almost a third of the city wasn’t even born within the US.