Not exactly. Northern Blacks started to fall for the swan song of the Democrats in the mid to late 1930s, and they were useful idiots for Democrat Urban Machines.
Southern Blacks were largely disenfranchised, and the few places they were allowed to vote were also largely controlled by Democrats, though they were more Republican than Northern Blacks, but that ceased after the early 1960s. Nixon got about a 1/3rd of their vote in 1960, but it fell to practically nothing (single digit %) in 1964 and hasn’t been appreciably higher than that in every Presidential election since.
David Benioff and D.B. Weiss doing their level best to start a race war...
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.