[Albion Wilde]

Here is an explanation and some charts illustrating your point. Although the reversal began with FDR, it wasn't "complete" -- both party identification and actual voting preference -- until Truman.

I notice that the source article from FactCheck is from 2008, so it leaves out the years that would have shown a surge in blacks voting GOP for Trump.

[ansel12]

Here is another chart that goes to 2020.