The 14th Amendment turned the three-fifths compromise into a five-fifths concession. Former slaves were fully counted as citizens for purposes of allocating House seats, and thus electors in the Electoral College. When Reconstruction failed and Jim Crow laws denied black people access to the ballot, the white southerners ended up with increased power in Congress and over the selection of the president.
If black people did succeed in voting, despite all obstacles, then their votes for anyone but the choice of the white majority were discarded under the "unit rule."
The Electoral College system was a tool of repression against former slaves and their descendants in the south through history until even today.