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.