Black sex link birds from hatchery don't have black legs. I'm thinking Australorp. The photo to me shows white feet which is not the hatchery sex link standard or yellow.
Did you breed these? Hatch the eggs under broody? Where did they come from? That would clear up some confusion. Black sex link males are barred and female black that leak red coloring, especially around neck, as they get adult plumage. Red sex link females are yellow when chicks.
The bird in question came from a farm. Black sexlinks are a cross breed so there is a huge range of what pullets may look like, depending on what breeds were crossed.