It may be a melanoma, which is malignant, unfortunately. If she dies eventually, you can get a diagnosis most times from having your state vet do a necropsy. Sorry. Melanoma in chickens has that very dark color that you can see in the following pictures:
https://www.researchgate.net/figure...ltifocally-replace-the-skin-and_fig1_51555713
https://www.researchgate.net/figure...ltifocally-replace-the-skin-and_fig1_51555713