I have no issue with what you said other than to point out that the switchover was relatively rapid after 1964. Yes, it began during Roosevelt, but it accelerated massively after 1964.
I was pointing out that 1932 was the last Presidential election in which the GOP received a majority of the Black vote. 1960 was the last in which they got over 15% of the Black vote.
The fictitious claim we see around here that “MLK Jr. was a Republican” is easily debunked when you consider he willfully led the remaining non-Democrat Black voters into their party in 1960 with his endorsement of JFK (despite Nixon being pro-Civil Rights) and in 1964 for LBJ (again, despite Goldwater being pro-Civil Rights, but opposing the ‘64 CRA for being a drastic and unconstitutional overreach by the government). MLK devastating and falsely labeled Goldwater and Conservatives as “tools of Southern White racists.”