Is black hairy tongue dangerous?

Usually not. usually it is self limiting and doesn't cause any complications . http://www.dermnetnz.org/topics/hairy-tongue/
no. Many people have black or even white hair tongue-some various causes-anemia-gastric upsets -heavy smokers -radiation treatment-not dangerous- brush tongue to keep it clean.

Related Questions

What is a black hairy tongue?

Overgrown papillae. The tongue is coated with taste receptors termed papillae. These normally stay short from food chewing and constant rubbing in normal activity of the mouth. If they grow longer, they can be stained darker by food and other substances in the mouth. It is not harmful and is not pathologic. You can lessen it by brushing the tongue as well as your teeth with your toothbrush. Read more...
Papillae overgrowth. The tongue is coated with taste receptors termed papillae. These normally stay short from food chewing and constant rubbing in normal activity of the mouth. If they grow longer, they can be stained darker by food and other substances in the mouth. It is not harmful and is not pathologic. You can lessen it by brushing the tongue as well as your teeth with your toothbrush. Read more...

How do you know when you are starting to get a black hairy tongue?

Tongue appears hairy. The hairy appearance is due to the elongation of keratinized filiform papillae, which may have different colors varying from white, yellow, brown, to black depending upon the involved extrinsic factors (ie, tobacco, coffee, tea, or food) and intrinsic factors (ie, chromogenic organisms in the normal flora). Read more...