I think they will go with the most predictable choice and make Faye Cassie's half sister. I read the books (and actually didnt quite like them, I think that the tv series is waaay better) and there wasnt any other Black's child in the circle. But im guessing that if Faye had dark magic, she would have already found out (she's impulsive and reckless, I think she would have use it unintentionally already).
Anyway, in the books there needs to be 12 witches in the circle to bind it, but it can be from any families (it can also be repeated, like 2 brothers or whatever), but it seems to be different here. Maybe they just need 6 witches from 6 families, but any of the bloodline works, so its not imposible that Diana has blackwell blood.
But maybe they didnt mean that there was another blackwell child, maybe they were talking about John being alive and all (I dont remember exactly how he said it).
I actually thought the witch hunter just spoke of another descendant and said that Cassie wasn't the only one with dark magic in the circle but I looked it up on YT and turns out I was wrong lol His exact words were "Cassie Blake is not the only child of Blackwell in the circle"