is very dark brown. I believe it started a year or two ago but I am not sure and I believe it happened after masturbaiting and it looks as if it is a friction burn
. If you are having hyperpigmentation in patches, then other possibilities need to be evaluated. Friction rub or chafing produces red, irritated skin and not brown skin.
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 dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing. Warm regards.