Hello,
This skin pigmentation can be due to hyperpigmentation or acanthosis nigricans. 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 which needs to be ruled out.
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.
You don't mention if this is something new. If it is, you might want to be checked for insulin resistance. You might want to point this out to your doctor so you are not overlooking something like prediabetes or diabetes. Otherwise, if it proves to be nothing, I'm not sure what would work to lighten the area. Sorry and hope you find the answer you are looking for. Good luck.