I mostly blame Reagan and Nixon. They were the harbingers of modern republican governmental stupidity. Nixon courted the racists out of the democrat party and Regan push dumbass deregulation. Bigotry + being the whipping boys for rich people is basically the only principles republicans stand on today.

Civil Rights happened.

Once it became clear to racist whites that Black Americans would have full access to the social programs that they enjoyed, they decided that they'd rather burn all those social programs to the ground before they'd share them. This is the basis of the modern Republican party, so you weren't wrong.

Incidentally, the scenario in this original post was never true for almost all black Americans.