There are many reasons for the black color on the inside of the tongue, such as physiological factors, digestive system disorders, infectious diseases and so on. 1. Physiological factors: Eating food containing dark color pigment, such as mulberry, betel nut, etc., may cause the tongue to be stained, thus leading to black tongue, which is temporary staining, not a disease. 2. Digestive system disorders: digestive system disorders can cause the intestinal absorption function to decline, so the food stays in the gastrointestinal tract for too long, which can lead to acid reflux, belching, and thick and greasy tongue, and thus there may be a piece of black on the inside of the tongue. 3. Infectious diseases: the tongue itself is infected with pathogens such as fungi and bacteria, and if the flora is colonized on the tongue, it may lead to the phenomenon of black lumps on the inside of the tongue. There are many reasons that lead to black tongue, we suggest you to go to the hospital in time to improve the examination after symptoms appear, and give standardized treatment after clear diagnosis.