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The condition what you have is called ACANTHOSIS
NIGRICANS(AN) ,in which one has thick and black skin over neck . Knuckles of fingers might also be involved in certain cases.This might be associated with over weight or can be familial.
This is not fungal infection
(in which one would give history of itching).
AN is resistant to treatment and there are very few options .The first being reduction of weight. One should start with tretinoin
cream locally at night over the area ,and a sunscreen in the morning. Also creams like hydroquinone
can help a bit. If this doesn't work then the latest is use of one anti diabetic tablets .For all this you need to consult a good dermatologist