Very few blacks voted democrat during FDR's tenure. Democrats were the party of slavery. Republicans were the people who crushed the party of slavery. That was still very fresh in the minds of American blacks. FDR was probably the greatest snob in presidential history. I think Truman had twice the guts FDR ever mustered. Truman and Kennedy were the last great democrat presidents. We haven't seen another since.
Truman and Kennedy had slaves then?
Republicans crushed if anything the
nation of slavery, which is IMHO why that nation stayed Democratic for so long after that event, but the Democratic Party was for that period a bipolar freak, keeping one foot in that and the other in northeastern Liberalism, trying to be two different things to two different factions, with predictable results --- among which were "Dixiecrats" walking out on Truman's convention, one of several times that happened. Too much noisemaking about civil rights and equality for the Southern faction's tastes, and Truman had integrated the military which did not go down well with them. In 1964 they finally bolted for good, but for the previous century the DP may as well have used as its icon the face of Janus.
But slavery itself started waaaaaay before either party -- before any party in this country existed. Before the country itself existed, so there was no "party of slavery". It wasn't a political phenomenon. Nor was it in any way limited to this country or this continent.
Fun trivia fact: in the election of 1860 the Republican candidate was of course Lincoln and the Dem candidate was Stephen Douglas.
How many states did Lincoln win in the South? Zero.
How many did Douglas win in the South? Zero.
Serves to underscore what the driving forces are, and it ain't political parties.