What Causes Dark Spot On Knee?

Hi! I have a dark spot by my knee that has been there for about 3 years. I itches a tiny bit once in a while, but goes away. There is no irregular borders, no pain, no flaking, no tenderness to the touch, nothing, just like a brownish healing bruise, that fades close the borders. Actually I noticed when I looked at myself in the mirror and I compared to the other knee. I take birth control pills.Thank you.