The US African-American population only achieved political equality with the Caucasian population in 1964.  (Legal protection is the sina qua non for wealth accumulation.)  Prior to 1964 lynchings, riots - the destruction of black owned property by whites, 'red lining' by banks, etc,. etc., etc., kept blacks from accumulating property - defined broadly - in one generation and, thus, providing a basis for the next generation to 'add their bit.'  Although there were exceptions, there always is, these exceptions were limited by overt or covert Jim Crow laws to the AA population, that didn't have a lot of spare cash to purchase, thus limited capital accumulation to/for black owned businesses.

On the macro-level, the Northern Migration from the South to northern cities during WW 2 and the two decades following occurred exactly at the time the US gutted the basis and economic value of cities -- white flight, suburbanization, & etc.  Thus black micro-economic activity in the north was located in an area that was in decline.  Blacks found themselves purchasing assets that were losing value.

Blacks that did 'make it' where heavily involved in 'scalable' industries where a small percentage of the people involved grabbed a majority of the money in that industry: entertainment, sports, & etc.  

To be sure some of the above were affecting poor and working class whites as well.  But the Post-WW 2 trends impacted blacks more and worse because they were starting from a lower economic position.