Black Tongue

add on from web md
The name -- black hairy tongue -- may sound scary, but the condition is actually harmless. Black hairy tongue is caused by bacteria or fungi in the mouth, which make the tongue appear black and hairy. It's easily remedied by good old-fashioned oral hygiene.
What Causes Black Hairy Tongue?

A black hairy tongue is caused by too much bacteria or yeast growth in the mouth. The bacteria build up on tiny rounded projections called papillae. These lie along the surface of the tongue. Instead of shedding as they normally do, the papillae start to grow and lengthen, creating hair-like projections. They can grow to 15 times their normal length.

Normally, the papillae are pinkish-white. But as they grow, pigments from food, drinks, and possibly the bacteria or yeast themselves get caught in the papillae, dyeing the tongue a color. Most often that color is black, hence the name. But the tongue can also turn brown, yellow, green, or a variety of other colors.

Certain lifestyle habits and conditions can make people more likely to develop black hairy tongue. They include:

poor oral hygiene
smoking tobacco
drinking a lot of coffee or tea
using antibiotics (which may disrupt the normal balance of bacteria in the mouth)
being dehydrated
taking medications that contain the chemical bismuth (such as Pepto-Bismol for upset stomach)
not producing enough saliva
regularly using mouthwash that contains peroxide, witch hazel, or menthol
getting radiation therapy to the head and neck