Odd Black Spots (note: pirates not to blame)

[Dys]

So, my beardie Roscoe has some rather curious dark spots/marking around a couple of his feet, and now one of his legs.

I'm unsure if this is some kind of parasite, or if he's just got some very stubborn dirt that is refusing to wash off, or if this is just something that happens with age that I don't know about! His limbs are mostly lightly colored, for the sake of my concern.

Here are some pics. Anyone want to help me out, here?

 

[mrsphate]

I'm not sure, mine seems to have a spot like that too, he is a rescue and I thought it was a burn mark.

 

[Mistyck]

What substrate is he on? Do you feed him in his tank? If so, what do you feed him?

 

[mrsphate]

I am wondering what this is too still because my Chuck has look too.