Thank you very much for your help!
Hello,
It can be due to hyperpigmentation skin, joint friction pads or addison’s disease. To confirm the diagnosis, please get it evaluated from a dermatologist. treatment is different for all.
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.