Well, even though your mollies are black mollies but one of the parents may have an albino gene carrying. For you don't know if an albino has crossed in the bloodline of one of the parents. But if both are phenotypically black eyed black molly, and there's albino fry in the batch of fry, you need both parents to carry that albino gene. It's a recessive trait. If only one carries that albino gene, all fry should be phenotypically black eyed.
If you have purchased them recently, it could also be that the female has mated with another male before and used those stored sperm packets of that particular mating and not from the male you've got currently with her.
If you cross a black molly to a non black molly, part of the fry will be totally black, the color of the other parent and a mix of both. But if you put a phenotypically black molly fry from this mix with other pure black mollies, you won't be able to tell the difference.
In general, black mollies will leave their fry alone.