There are many reasons for the blackening of the tongue, including two categories of physiological and pathological reasons, of which the pathological reasons are chronic diseases, infections and so on. 1. Physiological reasons: for example, the staining of the tongue occurs after eating dark-colored food, such as mulberry. 2. Chronic diseases: such as renal failure, cirrhosis of the liver and other diseases, resulting in circulatory ischemia of the body, which may also lead to the blackening of the tongue. 3. Infection: mostly seen in patients with high fever. The blackened tongue is the result of fungal infection of the tongue in the mouth. If there is a black tongue, first of all, we should pay attention to it, to clarify whether it is physiological or pathological reasons, it is recommended to improve the relevant laboratory tests in the hospital, under the guidance of the doctor for treatment.