Reasons for a black tongue include smoking, too much tea or coffee consumption, antibiotics and dehydration, according to WebMD. Other causes of a black tongue are bad oral hygiene, taking medicines that contain bismuth, dry mouth and radiation therapy to the head.
Another cause of black tongue is the regular use of a mouthwash that has menthol, peroxide or witch hazel, says WebMD. In these cases, the tongue is not only black, but hairy. Men who have HIV infection, and use intravenous drugs are more at risk for getting a black, hairy tongue.
In most cases, the remedy for a black tongue is proper dental and oral hygiene, says WebMD.