Except for a few states like Vermont, most states and most cities have significant black populations. See, for instance: http://fivethirtyeight.com/features/the-most-diverse-cities-.... Even in spite of that segregation, people do talk to each other (they may not live on the same block, but they do eventually meet each other).
Do you live in the US? If you don't, please do try to get basic facts about us right. Our country has a lot of problems, but you can't understand them without having a basic factual grasp of our demographics.