In 1960 Nixon received thirty-two percent of the Black vote. If Republicans received that amount today, we’d never see a Dem president. What happened? Among other things two strong voices of the Republican/conservative side, W.F. Buckley and Barry Goldwater, were against the passage of the 1964 Civil Rights Bill. Whatever one thinks of the merits or demerits of the bill (and despite the fact that more Republicans voted for the bill than Dems), it cemented in the minds of many black voters that Republicans were racists and their party was the Dem party.
Sadly, it was Goldwater that marked the low point for support (even despite the fact that he was pro-Civil Rights, he just opposed aspects of the CRA for being overreaching).