The black vote switched over permanently in the 1936 election.
1932 was a typical black vote of 23% for the democrat, since blacks had always been republican, but for 1936, the switch was total, the democrat got 71%, and it has been so ever since.
Eisenhower did get almost 40% of the black vote in 1956, but you’re correct, the trend had been for almost total Dem domination of the black vote for many decades.