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.
Didn’t Eisenhower win the majority of black votes in 1956?
Eisenhower got 40% of the black vote. Johnson got 94%
.