Blacks don't get cancer? They wouldn't use renewable and efficient energy sources? You read all that about all the potential innovation we lost out on because of the deaths of 650,000 Americans and their posterity that never existed, right? Blacks would have benefited from all that just the same as whites.
Maybe this is a better way to put it....say slavery existed but it came to a peaceable end because there was never a fugitive slave act and it was unsustainable. The blacks runaway, live in the north, and never have to face a post-bellum south. The KKK is never founded, because there are no disenfranchised southern whites. Jim Crow never happens because southern whites don't turn the ire of losing a war on blacks, etc. Without the civil war, the southern treatment of freed blacks would have looked a lot more like the northern treatment of freed blacks.