That was really always a non-starter. Consider the horrific circumstances of the Middle Passage, and that had the rum-molasses-slave triangle to economically support what little care was given the slaves en route. The Blacks of America built the physical foundations of America, right down to her roads, rails, canals, sewer systems, bridges, and capitol. They were NOT going to accept a "OK, you're finished, now get out!" message. After generations of toiling to support their White masters in the lap of luxury, they were quite ready to support themselves on their own, thank you very much.
Besides, there were four million freed ex-slaves. Moving that number of people, even to the Caribbean-nevermind Africa-would have bankrupted an already near bankrupt USA after the costs of the American Civil War. Did they expect the freedmen to pay for the journey themselves? And anyway, the Caribbean islands were overpopulated as it was.