I have used a test tube type container as a vaporizer, it definitely was glass because I eventually dropped it. It made the same type of black spots when I heated it. I wasn't especially worried because I was heating the outside but it probably was some type of film, or perhaps the gas and other chemicals from the flame are accumalating on this type of glass.

The film reminds me of the blackness that sometimes accumulates on glass candle holders. I have always figured a chemical in the wax caused this, but I'm not sure. I'd do more research or find another use for it other than a bowl.