What is the black substance dentists place on canker sore?

Debacterol. There are different products that can be used to help with the discomfort of canker sores. Everything from a topical anesthetic, laser energy, silver nitrate, sulfuric acid etc... One common product that is black is called debacterol. Hope this helps, .
Debacterol. Debacterol is a great canker sore treatment. It uses chemical cauterization of the wound. It is extremely effective.