This is an incredibly complex issue that touches on everything from mortgage lending policy to transportation to industrialization to slavery and jim crow.

Arguably the most important element is that it was simply difficult for blacks to buy property outside of cities. Racially restrictive convenants on many housing deeds meant that you couldn't sell your suburban home to a black person. That was (ostensibly) a way for developers to make sure blacks didn't move in and drive down property values. The fun part is that these covenants are still on lots of deeds, though are legally unenforceable. Plenty of blacks live in homes where the deed says they're not allowed to live.

Couple that with lending policy, which allowed (and in some cases still allows) for mortgage rates to be MUCH MUCH higher in already black cities than in white suburbs. The logic behind this is that inner city property values are more volatile (especially given racial unrest in the 1960's), and thus a worse investment.

So blacks couldn't afford to buy homes in cities because of the mortgage rates, yet they weren't allowed to buy homes in the suburbs. Low ownership, lots of slum landlords, etc are the result. Which really slams property values.

At the same time, through the 1950's and 1960's we're building highways like crazy, opening up TONS of suburban land for development. Blacks weren't necessarily banned from these areas, but they sure weren't encouraged to move into the 'burbs. Whites bought the tract homes and moved out of the cities, further depressing property values and fucking over blacks.