Yep, figured this all out when I made a pinhole out of a tin can with a shiny inside - got the weirdest "lens flare" on the negative you could imagine. covered the inside with black gaffer's tape and problem was solved.
My Calumet 4x5's bellows are gray on the inside, but I guess the corrugated nature of the bellows helps scatter and absorb internal reflections. I can't help but wonder if I could lower the base fog of my Negs by spraypainting the insides black or something. Probably jsut ruin my bellows.