What Causes Little Black Spots Around The Knees?

I was wondering what these little black dots are.. They look like little bumps but they dont look like warts or feel like it. Theyre smooth.. What could they be? Ive got them around my knees. Could it be a wart? Because i also play alot of sports so could it be a sport related injury? I dont know because ive had them for a while. But they dont hurt