What Causes Black Substance On Penis Area?

I'm 20 years old, 5'8" and I'm not so sure how much my weight is.. but I'm kind of chubby. I've been having a problem. I've noticed black "stuff" on my genital area and I don't know what it is. It just brushes away easily, however I've also noticed a smell. Nothing like a fishy smell or anything; but it's getting me concerned.

Also, I am circumcised and I am still a virgin; so I no it can't be an STD.