In Muslim society, one can choose not to believe in Islam and have no problem, as long as one continues to outwardly perform the expected public rituals, recite the shahada etc. (Quiet personal atheism is much more widespread in the Muslim world than outsiders might suspect.) But if you are a black slave or a descendant thereof in a society based on blood purity, you can’t change your skin colour or descent no matter hard you try. So, while both bases for slavery and segregated society are indeed bad, it’s hard to claim that one might not be a more preferable fate for some unfortunate people than the other.

This is a bit of a non-sequitur? You couldn't just "choose to pretend to believe in Islam" and somehow escape slavery. And with castration having been a common practice, anything that was dependent on descendants was going to be different at face value. So, kudos to them for not allowing their slaves to continue their bloodline?

Look, I'm open to the idea that slavery in the US was somehow the worst slavery ever. But most arguments used to prop up the idea rely on laughably naive views on slavery elsewhere. Worse, much of it seems to stem from a naive "US bad" gullibility in taking in information.

Um, you might want to ask certain minorities about that.