All the African-Americans who hadn't already joined the Democratic party fled into its arms in a riotous rush.
Actually, the reason all those blacks rushed to the Democratic Party is because the vast majority of them lived in the South at the time, and well, back then, voting in a Republican Primary was still a wasted vote. Remained a wasted vote until last decade. It took voters a long time to realize the parties had switched, and it was primarily a generational thing.
My grandfather never voted for a Republican in his life, even after L.B.J. He still viewed the Republican Party as the “party of Lincoln”, even though all evidence suggested otherwise. Then again, he was also born during the Teddy Roosevelt administration. Once again, that was entirely generational.
As it becomes clear that black votes are losing their power in the South because Republican primary winners seem to always win election, you’ll begin to see blacks convert, if just to preserve their vote.