The correct term for tooth decay would be caries, and the entire carious tooth turning black could be the result of pulp gangrene, in which the necrotic tissue of the pulp is broken down by decaying bacteria to form black iron sulfide, which gives the necrotic tissue a black color, and the color penetrates through the enamel to make the entire tooth look black. Untreated caries will continue to progress toward the pulp cavity, where bacteria enter the pulp and cause pulpitis, eventually leading to necrosis of the entire pulp. Some of the pulp necrosis is accompanied by a putrefactive bacterial infection, which breaks down the necrotic pulp tissue and produces hydrogen sulfide that combines with the iron in hemoglobin to form black iron sulfide, resulting in a black coloration of the necrotic tissue. There may be other reasons for the blackening of the entire cavity, so it is advisable to go to the hospital in time.