A black tongue is usually a harmless condition that can be caused by medications, smoking, poor oral hygiene, soft diet, or dry mouth. A black tongue is usually associated with elongated tongue papillae and thus, it is called black hairy tongue  (lingua villosa nigra).The cause is thought to be a change in the normal bacteria in the mouth after antibiotic treatment or use of products that contain bismuth such as Pepto-Bismol. black tongue is usually a harmless condition that can be caused by medications, smoking, poor oral hygiene, soft diet, or dry mouth. A black tongue is usually associated with elongated tongue papillae and thus, it is called black hairy tongue  (lingua villosa nigra).