Hello,
One possibility for your symptoms is acanthosis nigricans. This darkening can also occur when an excess of melanin, the brown pigment that produces normal skin color, forms deposits in the skin.
Acanthosis nigricans presents as symmetrical, hyperpigmented, velvety plaques mostly in the folds of skin. It is not itchy and is characterized by hyperpigmentation. My sincere advice is to consult a dermatologist. The other possibility is of Addison’s disease.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Are you overweight. Possibly "Benign Acanthosis nigricans".