Around 1938, black Americans identified as Democrats as often as Republicans.
However, in 1949, Democrat Harry Truman appealed for new civil rights measures from Congress; voter protections; a federal ban on lynching, and enforcing existing civil rights protections. That year, more African-Americans identified as Democrats.
The next jump of African-American Democratic party loyalty happened with the 1964 Civil Rights Act, in which Democrat President Lyndon Johnson said the South lost generations of the African-American vote. It wasn't only the Civil Rights Act, though that was enough to move blacks to the leader of the party who signed the Bill, it was the Southern racist Dixiecrats crossing over to the Republican Party, which showed the Republican Party no longer welcomed people of color.
At no point beyond 1936 did the Republican Party receive more than 40% of the black vote. In 1968 another spike in Black Democratic ballot box left Republicans with token black voters.
Essentially, the Civil Rights Movement and the South, who used to vote Democratic, began voting Republican. Supporting a conservative agenda, with racial undertones and servants to big business rather than to the people elected to represent.