In the Emerging republican majority Kevin Phillips pointed out that the South did indeed have a higher GOP proportion of blacks than the north. But, Not a big minority much less a majority. They were Dem’s to from my recollection.
As I said, the Dems pretty much unified with their Northern brethren once the ‘64 election occurred (however, some Southern Blacks weren’t as uniformally hostile to supporting some Republicans — indeed, in my state, a Black Dem crossed over to elect the first GOP House Speaker in TN in 1969 in a century. They also were the deciding factor in electing a GOP Mayor in Birmingham in that era and tossing the Seg Dems that had controlled the office to that point).