Excess bacteria or growth of yeast in the mouth causes black hairy tongue, according to WebMD. Papillae are very small, rounded pinkish-white projections on the tongue's surface. When bacteria builds up on papillae, they grow long, absorb pigment from food and other sources, and turn black.
Several behaviors and conditions can lead to black tongue, such as smoking tobacco, poor oral hygiene, frequently using a mouthwash containing menthol or peroxide, and becoming dehydrated, states WebMD. Intravenous drug users and people who are HIV-positive are more likely to develop black hairy tongue. Generally, black tongue patients do not experience discomfort. A good oral hygiene regimen, including brushing the teeth and tongue twice a day and drinking lots of water, is the best treatment.