Why is the roof of my 5 year-old's mouth black?

I just noticed that the roof of my 5 year-old daughter's mouth has black stuff on it. I noticed some on her gums and a few specks on her tongue and even a couple on her teeth. The black came off with some rubbing with a wash cloth and her tooth brush, with the exception of a little in the crease in the roof (which may be because she made me stop brushing). I am wondering what would cause this and if I should be concerned?