When all its white citizens weren't forced out by blacks
this was literally never true
what happened was home sellers wanted to find a way to sell these hip, new "suburban" homes, so they used a practice called "blockbusting," leaving whites to sell homes in fear of black or other race neighbors, leading to white people being clustered in the suburban area of urban development
if you think that black people "forced out" white people from urban areas at any point in history, you'd be completely wrong