Go to the grocery store and get some black food color, mix a drop or so in with some clear Karo syrup. Put it in your mouth and swish it around for a min. Just keep in mind that your mouth and anything it touches WILL BE BLACK.
The only bad thing about black food coloring is that it stains and may take a while to come off?!. LOL I just purchased some "Tooth FX" by Mehron at my local Display Costume shop for $5.50. It's alcohol soluble and brushing your teeth gets the stuff off. They have different colors for whatever costume you are trying to achieve. I saw the black, blood, and yellow/nicotine one. I bought the nicotine one as I didn't want completely black teeth. HTH!