It is really not that perplexing. The downward trend coincides with the rise in popularity of the civil rights movement in which blacks were convinced they were a victim of the white man's society, coupled with the ever popular welfare system and affirmative action plans.
Yes, my grandparents must have had the fantasy of living under Jim Crow implanted by the Democrats. Legal segregation did not really exist.
The truth is, it is a lot harder to build a strong family without two parents and two incomes. A single mother is going to have a hard time, even if she comes from a middle class background. When 70% of black children are born out of wedlock, it isn't surprising that economic progress has began to reverse.