Early on I know that the black vote went for the 'Party of Lincoln', and I recall that this was at least the case through the turn of the century, but was the switch in party support mainly brought about by the push by the Democrats for the Civil Rights Act, or had the shift already mostly happened by that point due to the changes in the northern wing of the Democratic Party?
This shift of the African-American vote occurred in concert with the ideological sorting of the parties during the modern presidency (FDR onwards). Even during the end of Jim Crow and the passage of the CRA the party system was a confused mix on both the Democrat and Republican side - neither could fully claim to be the liberal or Conservative party, as each had elements of both. However, during the 60s and 70s the liberalization of the Democratic Party and the sorting of conservatives in the Republican Party accelerated, and with that sealed the allegiance of the African American vote.