It started around 1932 because FDR was for "poor people" and his message appealed greatly to blacks because many of them were poor.
By 1974, most of them were voting Democrat.
Were the 1960s Democrats segregationists?
The Southern ones certainly were according to all the movies i've ever seen, and I suspect the Northern ones were too, but we don't see those kinds of movies.
The Republicans probably also were. Bear in mind, it was liberal (FDR/Truman) appointed courts that overturned all that.
Were 1960s labor unions Democrat campaign funders?
Are 2020s labor unions Democrat campaign funders/