Rand Paul recently gave a speech in which he asked "How did the Republican Party, the party of the Great Emancipator, lose the trust and faith of an entire race?" Charles Blow replied that this was an easy question, and offered a number of causes beginning with Richard Nixon's "Southern Strategy." Oddly, he left out the first and most important: the 1964 presidential election, when the Republicans nominated Barry Goldwater, who had not only opposed the Civil Rights Act of 1964, but denounced it as unconstitutional.
According to the American National Election Studies, the Republican candidate averaged 27% of the black vote in the 1952-60 presidential elections, and 0% in 1964. Of course, that's just the ANES sample, which included only 94 blacks in 1964. Goldwater undoubtedly got some black support, but it was almost certainly less than 5% (if he actually got 5%, there's a 99% chance that he would get at least one vote in a random sample of 94 from that population).
Blow isn't alone here. Goldwater's nomination is widely remembered as a key moment in the development of modern conservatism, but its role in the development of racial divisions in voting often seems to be overlooked. If the Republicans had nominated someone who had supported the Civil Rights Act, as the great majority of leading Republicans did, they would probably have much better support among black voters--not as much as they'd had in 1952-60, but enough to maintain a foothold. As far as why the 1964 election has been overlooked, it may be because Goldwater had rehabilitated his reputation pretty effectively by the late 1970s. He was out of the Senate between 1964 and 1968, and after he returned he took on an "elder statesman" role (for example, endorsing Gerald Ford over Ronald Reagan in 1976 in the interests of party unity). Nixon was discredited by Watergate, so he made a better villain.