I mean, the black-maskers are basically cultists; the fact that the story they were fed is technically true is kinda secondary to the fact. They're the Lambs of Dalamud with a nicer robe cut. I agree that the reason the Big Boy Ascians gave them black masks instead of white probably is a sort of instatement of inferiority; we know that they think sundered people are lesser, so why would they give them a dress code that implies they aren't?

Thinking about that, my question is less about the black masks are like that, I'm wondering why exactly the Big Boy Ascians landed on the design they did for their own robes. And to a bit of a more relevant degree, why is it that the one person deemed worthy of a white mask post-sundering was Unukalhai?