My first thought is it could be ringworm...but generally ringworm has a raised area. Ringworm isn't a worm at all...it's a fungal infection. A vet would prescribe ointment to clear that up. Ringworm can also be spread to other animals...including people.
Looking at the black area...may indicate she was bitten by something and the skin/flesh has died in that area.
I would definitely get her to a vet ...
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