Darkening of the middle of the tongue indicates that the tongue coating is black. Whether blackened tongue is serious or not needs to be analyzed on a case-by-case basis and should not be generalized. Black tongue coating may be due to bacterial or mycobacterial infections. Disorders of the gastrointestinal and digestive tracts can show color changes on the tongue. The center of the tongue can also be blackened by chewing some pigmented foods such as mulberry and betel nut. There are also some rare cases, such as when there are some diseases in the human mental system and immune system, some black tongue moss will appear on the surface of the tongue. All of the above are benign lesions and are not serious. While a small number of black tongue may be the middle or late stage of malignant tumor, indicating further deterioration of the condition and a more critical situation. If the blackness in the middle of the tongue is only caused by food, there is no need to worry, but if the black tongue is accompanied by discomfort in the gastrointestinal tract, liver and other parts of the body, it may indicate the presence of some corresponding diseases. When such symptoms occur, you should seek medical attention for early detection of the disease in question.