People self segregate-- I don't know if that's the top reason, but I'd say it's up there.

Numbers-- I'd guess not a lot of black folks can afford to live where a lot of white folks live (assuming it's not a dumpy nabe or anything) and if they could afford they might still choose a black neighborhood just out of comfort.

I think there's enough in those two reason right there. Are there neighborhoods that try to keep non-whites out strictly on gp? Sure, but we can't really speculate on how few or many there are. Personally I don' think that model is rampant.