Well, those fry could be fathered by more males. If the female has stored sperm packets in the folds of her fallopian tube, she'll release randomly those sperm packets to fertilize her eggs. So, she could've had a former mating at the store or even before.
But it could also be that one or both parents are genotypically not just black. And carry genes of a white molly influence that may result in white offspring. In both cases, this is normal.