I had what I thought was a permanent bruise on my shoulder blade. Turns out it is acanthosis nigricans due to insulin resistance. I have more area of skin colour change too such as under my arms. These areas may also be itchy. Very suspect if you are overweight or obese, eat a diet high in sugar or refined carbs, or are diabetic. Although some people can be thin with insulin resistance. Tests insulin serum. There is also a test called Homa IR. My issue was too much refined sugar from my diet. I now read labels!
Since you have had it so long, I would see a GP or dermatologist, it may be nothing, but then again, it may be caused by an entirely different issue. It is always best to have things like this looked into, especially if it has been going for a long time.