Severe/frequent itching really needs to be seen by a vet
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Is the "black areas" on her belly sort of look like little patches of tiny black spots? Does she have it in her armpits? Where is she itching?
This could be allergies, a yeast infection, the beginnings of pyoderma, diet, or a primary disease that causes skin issues as a secondary symptom.
Skin issues can be really tough on generalist vets...so I'd recommend a Vet Derm, if you can see one. They are truly wonderful when dealing with skin stuff!