I think the Proper Black guy is from a different family, while the white platinum hair is from the Targaeryen family. Now, supposedly these interbred with one another, but without changing the color of the black family. Just like in Dune, we had Fremen looking like Bedouins and Fremen looking like proper unadulterated black sub-Saharans - apparently, you're only allowed to marry someone of your own race among the Fremen.

It's so ridiculous. And no, it's not addressed. At least we don't get lectures about wacism - thank God for small mercies.