There are many reasons for a black tongue, specifically the following: 1, it may be food staining, such as eating dark foods like mulberries. 2, chronic diseases are more common, especially critical diseases, such as kidney failure and cirrhosis, can lead to a black tongue. 3, gastrointestinal disorders, mainly seen in patients who have suffered from gastrointestinal disorders and indigestion for a long time. Mental illnesses can also cause this, as many patients with mental depression can experience gastrointestinal dysfunction. 4. Infections, mostly seen in patients with high fever. In general, a black tongue is a fungal infection of the tongue in the mouth, suggesting a weakened immune system and mostly a concomitant manifestation of systemic or oral diseases.