Republicans freed the slaves. That is why the party was founded.
Republicans desegregated schools under Eisenhower and Nixon. Republicans voted overwhelmingly for every civil rights act up until the big one in 1964. All of them were opposed by a majority of Democrats. Segregationists supported Democrats including Hubert Humphrey and George McGovern. LBJ took credit for the 1964 Civil Rights Bill even though he originally opposed it and though it was opposed by a majority of his party. Martin Luther King was a registered Republican.
The fact that black people are now overwhelmingly Democrat is a triumph of brainwashing over reality.