What is that stuff make your teeth black with to show all the plaque?

The substance used to show plaque on teeth is called a disclosing agent. It is typically a dye or a solution containing a dye that adheres to plaque and other debris on the teeth, making them visible. Disclosing agents are often used by dentists and hygienists to help patients identify areas where they may need to improve their brushing and flossing habits. These agents are not harmful to the teeth and do not cause any lasting effects.