Please show me where the Constitution says there must be black districts.
The assumption is that black people are being deprived of rights unless they can vote for a black candidate who is a shoo-in to win. Since no district is 100% people of one race, that means the voters in that district who are not black have no right to a representative who might be attentive to their needs (since he will only care about “his people”). At least that seems to be the reasoning.