Darkening of the tongue may be caused by hyperpigmentation, black hairy tongue, malignant melanoma, etc., as follows. 1. Hyperpigmentation: Melanin in food or drinks may be deposited on the back of the tongue after consumption, resulting in blackened tongue, which usually returns to normal color in about 1~2 days. 2. Black hairy tongue: the filiform papillae proliferate and elongate in the form of black hairs, which may be accompanied by obvious bad breath, and the excessively long filiform papillae may irritate the soft palate and cause reflex nausea. 3. Malignant melanoma: the black spot on the mucous membrane of the tongue undergoes malignant change, which appears to increase rapidly, with increased pigmentation and radiating expansion, and ulcers may occur on the surface, and other symptoms. There may be other reasons for the blackening of the tongue, and it is recommended to go to the hospital in time to clarify the cause of the disease and then carry out targeted treatment.