Whats the dark patch of skin on my upper left arm?

I have this small dark patch(doesn't have a shape, its pretty tered) of skin on my upper left arm. The rest of the skin surrounding it is lighter skin color but the patch is a darker complexion. It also has a few hairs growing on it. The lighter part of the skin surrounding it doesn't have hair. I'm wondering if it's a birthmark, but i could be wrong.