As we all know, during that time, the country was divided north and south. Also, as we all know there are presidential, state and local elections. Unless you are Candace Owens or Lee Elder, blacks in Alabama, MS, GA, SC, NC, etc. did not vote for the racist democrats in state and local elections like George Wallace and their predecessors. During that time a lot blacks migrated to the northern states for jobs and education. Perhaps the numbers of black democrats in the north were a higher, particularly when they moved larger metro areas like Chicago, Detroit, Cleveland, etc. But what I do know for sure is that after Johnson signed the Civil and Voting rights bills, it was a wrap. Blacks left the Republican Party behind. Your chart does show that also. Growing up in the 70s and 80s in NW Indiana, I don’t know anyone blacks that was Republican. That is still the case today.