What causes a blue tongue in humans?

Bad hygiene: Blue tongue may be due to build up of food debris under the structures on the surface of the tongue. Daily brushing or scrapping with a plastic device will help to keep the tongue clean. Congenital heart disease or some other forms of medical conditions can give you blue tongue. Consult with a medical doctor to do further test.

Many causes: Bleeding, central cyanosis, congenital heart disease, contusion, copd, lung disease or even a blue lollipop are some causes.