Thomas Sowell: “The black family, which had survived centuries of slavery and discrimination, began rapidly disintegrating in the liberal welfare state that subsidized unwed pregnancy and changed welfare from an emergency rescue to a way of life.”
The black vote switched in 1936 and never went back.
In 1932 FDR got the usual democrat black vote of about 23% but he did something that reversed the black vote in 1936 winning 71%, and it has stayed that way ever since, there was no gradual movement, it happened instantly and totally.