Blacks voted Republican for most of their history after the 14th amendment. It wasn't until the mid 60s that blacks started voting heavily for Democrats.
I think by 1936 FDR won the majority of the black vote—mostly residents of Northern cities, because the blacks living in Democrat-controlled Southern states were largely disfranchised until the 1960s. Southern blacks moving to Northern cities in the early 20th century faced some of the same problems (maybe more severely) as foreign immigrants did and supported the Democrats for the same reasons the immigrants tended to.