Foods that contain dark dyes can stain the tongue black. Other causes of a black tongue include tobacco use, oxidizing or astringent mouthwashes, and medications that contain the element bismuth. The U.S. National Library of Medicine states that a black tongue is harmless.
Foods heavy in dark-colored dyes turn the tongue black by staining its skin cells. Bismuth-containing medications such as Pepto-Bismol turn the tongue black when bismuth reacts with trace amounts of sulfur in the mouth. This temporary black stain can persist for several days after taking the medication, but eventually fades.
Tobacco and mouthwash can turn the tongue black by causing a condition called black hairy tongue. With this condition, the tongue appears black and bristly. Black hairy tongue may also cause a metallic taste in the mouth, a gagging sensation and bad breath.
Black hairy tongue occurs when the chemical balance in the mouth is upset. This allows bacteria and yeast to grow on the tongue. Other causes of black hairy tongue include antibiotics, dry mouth and improper oral hygiene. Black hairy tongue is not dangerous, but sufferers may find the symptoms unpleasant. It is treated by killing the bacteria and yeast with antiseptic mouthwashes or dentist-prescribed medications.