I do have some boys in the same hatch that are also looking a darker brown then her but when they hatched, they were a different color. she was tan and they hatched brown. but of course I always look at the boys and say are you really black and not brown? but the girl is definately not black. in the picture, girl is on the left, black boy on the right, notice the head color. she was all tannish brown when hatched. the brown boys are all dark when they hatch out.
I'm pretty sure the "Chocolates" that Tony was/is producing are actually Dun birds, not true sex linked Chocolate. I'm betting your girl is a Dark dun hen. The way to figure it out would be to breed her to a Black rooster, if she is Dun 50% of her offspring will be Dun and the other 50% will be Black.