If you don't have any black teeth wax on hand, the next best thing to blacking out one tooth in your mouth, or creating animal fangs, for a Halloween costume is eyeliner. Just make sure it's waterproof, non-toxic eyeliner, and you'll need to use Vaseline to seal it properly. Of course, you won't want to eat while this is applied, or it'll come off. It rubs off very easily when it's time to get rid of your black tooth/teeth.
Any questions? Let me know! I'll be happy to help.