White/Black Silkies

I have a trio of unrelated white silkie bantams which have been breeding well since last year with 3 sucessful hatchings. This time they produced 7 chicks, 6 white and one black. She (for I think it is a hen) is 4 weeks old and is jet black, eyes/beak/feet (5 toes)/skin and feathers apart from one tiny little patch of white on her chest.  Has anyone ever come across this before???