She definitely looks like an old english game bantam to me, but I'm not sure what would cause that dark skin. Is she still fairly young? If so it might change as she gets older. If not...hmmm, maybe she's crossed with something? Or maybe that dark skin just occurs in some blues?
I just raised a bantam sumatra pullet from hatch to now over five months old, and I really don't think she's a sumatra. Maybe, possibly, some sumatra in there, but not a sumatra. Definitely part or whole old english game bantam, I'd say.
Looks like and Old English Game bantam to me as well. Besides the face is the rest of the skin under the feathers black? OEGB should be white, if you are concerned about the legs, not all color varieties of OEGB have white shanks, blue/black/splash is one of them. Also, hatcheries aren't very particular about their breeding, so not all birds are breed to the standard.
Sumatras should have yellow skin.