I love this and will be thinking and stewing and may reply again. But for now I would add that I, too, assumed that Blaise's mother is a POC, but someone else pointed out that we don't know that for sure. Blaise could have a white mother and very dark-skinned father and still have an appearance Harry would call "black". But as you point out, we simply never meet her. Although I believe that in Jo's mind Blaise's mother is a POC and that all your points are valid. Much like with Hermione, there is what canon allows and what Jo intended.