This happens to me 90% of the time. Either black or light brown like the skins from popcorn. They come out when I spit though. I brush my teeth like a wild man, and at first I figured I was missing spots, but there's no way that's the case. I can't feel them when I'm pulling, and I am SURE that it's not residue in my mouth. My teeth are so tight that I have to fight just to get floss in there, and due to that, I can tell instantly when something gets stuck. I was always scared to ask : )