Hello sir. There are around 5-6 criterias which we consider when we talk about a pre existing mole or birthmark becoming cancerous or something to be worried about. If you are saying its persistent since last few years, then there is less to worry. But keep observing it for change in colour, margins, discharges or any peculiar sensations. If you are finding it doubtful, as u refered to it being sore and getting very dark, then do send me a picture by text and give the basic history, for further clarification. Thank u.