Northern Blacks moved Democrat after 1932, but were not voting overwhelming en bloc (for example, when Republican Oscar DePriest lost reelection to his IL Congressional seat in 1934, the Black Democrat who beat him still received only a minority of the Black vote and was elected by an overwhelming majority of urban Whites in that district).
Most Southern Blacks were disenfranchised and would remain Republican until the 1960s.
In 1932 Roosevelt got 23% of the black vote, in 1936 the switch was made, and it was permanent, in 1936 Roosevelt got 71% of the black vote while the GOP got the 28%, a complete reversal.
The democrat presidential races never had to sweat the black vote again.
