Well, it should be impossible to get a chick with slate legs from yellow legged parents, let alone getting a black barred chick out of barred duckwing parents.
So you’re sure they hatched from cream legbar eggs? And no rogue cockerel/rooster got into the pen at all? All the other cream legbar hens have the correct yellow legs?
Do you have pictures of the possible parents? Also, are you sure that another chicken didn’t breed with one of your legbars? She appears to have slate legs, which are incorrect for legbars. They’re also the result of a dominant gene, meaning one parent would also have to have slate or pink/white...