maybe it's a form of suntan - ie the skin producing melanin because something's irritating it. (Doesn't have to be sunshine.) I had that once - can't remember whether I found out what was producing it but it eventually faded. Sqad's view is no doubt better informed than mine.
My mum has a pigment patch on her forehead that started itching and flaking recently. I thought it was basal cell carcinoma which I wasn`t that bothered about as it`s not invasive. Her mother had the same thing on her nose and it was removed. If I had that, I would want it removed sooner rather than later so that it`s not too disfiguring. My ex boss donkeys years ago had a melanoma on his arm. I nagged him about it for about 18 months before he went to the doctors. He told me it was a mole but it was very clear that it wasn`t. It was like someone had stuck a piece of black chewing gum on his arm. He was very lucky because he had it for ages before he did anything about it.