Obviously black slavery wasn't "genocide" - they created majorities throughout the US south.
But it's also considered racist to point out 9/10 of American slavery was outside the US - not to excuse the south for anything, but to point out how huge the problem was, how badly it affected history of the Americas, and quite a bit how imbalanced we are in addressing the historical tides of this awful 500 year ugliness. When we make 1 sector in the US the only victim, we lose a lot of the lessons and diversity that came out of the period - how does societal evolution involving race look in the Caribbean & Brazil among former slave populations, what have they managed better or worse... Instead we again have American exceptionalism. Though seems the Brits have a bit of it as well.
And every racial comment has someone with a hair trigger waiting to be irreparably offended.