reegis

Hey all, I have 2 Epi Les Pauls with the same brand/type of strings. When playing one of them, I get black stuff on my fingers, even with brand new strings. I have tried cleaning the fingerboard many times and still get the stuff on my fingers. I think it may be coming from the frets because when I rub my fingers across each fret, I get the black stuff. My ques is what can I do to clean the frets and get rid of this stuff? Thanks!