The declining black population in San Francisco gets the front page treatment in today's San Francisco Chronicle. Having lived here for over 7 years (!) this is certainly not new news. I've lived by Union Square, in Noe Valley and Russian Hill over that time span, and I don't think I've ever had more than a few black people on any given block that I lived on. San Francisco's major industries (banking, law, advertising, medicine) don't typically have large numbers of African-Americans among their ranks (though my sister and several of her friends are attorneys).
As is mentioned in the article, there's really no easy way to fix this (if it can be fixed at all). The Bay Area is prohibitively expensive in many ways and there's no simple way to "create community". Communities grow through a largely organic process and an important part of any community is its shared history. Historically black neighborhoods like the Fillmore have seen the many lively jazz clubs and other haunts close in favor of upscale restaurants and botiques (this isn't about to turn into an anti-gentrification screed; the influx of the middle class can do good things).
The only thing I can think of off the bat would be for the city to aggressivly promote affordable housing initiatives to see if that would spur any action.