If you know anything about trees in the forests, they go through a process called succession. One species of tree will grow for a while, but then another will take over, and then the next. Then there will be a great fire and things start over.
When the all-black neighborhoods reduce themselves to post-nuclear-war landscapes with only a few residents remaining, developers begin to buy up the city blocks nearest the downtown business center and the neighborhoods begin another fresh up-cycle. Just like the forest.
It’s all whitey’s fault, of course.