My Uberti turned a white cotton Tshirt brown when I first cleaned it, right out of the box and freaked me out a bit too, but it was only the first cleaning.
My Uberti did the same thing. But you know I don't think they clean the barrel after they test fire them at the factory. Because when I took mine out of the box and looked down the bore with a bore light you can clearly see powder residue in there. They probably just fire them, oil the snot out of them and slap em in that bag.