Black tongue coating may be caused by dyeing moss, kidney yin deficiency and other reasons. 1. Dyeing: Black tongue coating may be caused by dyeing, when consuming black dyeing foods such as mulberries, black tongue coating may occur if the pigment adheres to the tongue coating, and this condition usually does not require treatment. 2. Kidney Yin Deficiency: If the patient’s tongue is thin and thin, with black and dry tongue coating, or accompanied by lumbar and knee soreness (a feeling of soreness and weakness in the lumbar and knee areas), heat in the hands and feet, dry mouth and throat, etc., then it may be due to Kidney Yin Deficiency. If you have a black tongue, you should go to the hospital in time to clarify the cause, if it is pathologic, and under the guidance of your doctor.