What causes black tounge?

Many possibilities : Smoking, poor oral hygiene, dry mouth, certain medications, overly soft diet can all cause black hairy tongue. Although unsightly the condition itself is harmless but can cause bad breath. To treat brush or scrap your tongue gently when you brush your teeth and eliminate the causative factor. It should resolve in 10-14 days. If caused by dry mouth or medication you may need to see your doctor.

Black tongue fungus: Antibiotics can upset the balance between the resident bacteria on your tongue and cause a fungus to take over that can look black due to staining. It is also helpful to maintain very good oral hygiene when taking antibiotics or any time for that matter.