Did Republicans try to enroll the rising #s of black voters in the north and midwest and west during the Great Migration?
How were the first northern Democrats to do so?
African Americans were an essential part of Big Bill Thompson's Republican machine in Chicago. In 1928, Oscar De Priest, the first African American congressman from the North was elected (as a Republican of course) from IL-01 on the South Side.
OTOH, the logic that "a vote's a vote" was not lost on all Democratic politicians or machines in the North. In New York, Tammany Hall had long cultivated the African American vote. Well before the Great Migration, both (Democratic) Mayors Carter Harrison in Chicago had wooed black voters with some success.
In general, though, it was pretty much taken for granted by both parties that African Americans would vote Republican. (A striking number of black intellectuals and journalists backed Al Smith in 1928, but had little effect on most of the African American electorate.)