A 50 year old Indian man suffered from a stroke that resulted in left-sided weakness. He was put on a pureed diet, which lead to a harmless, but bizarre condition: lingua villosa nigra. AKA... black hairy tongue. His tongue ended up with a thick, black coating that lasted roughly two weeks.
The condition, a result of a pureed diet, occurs when there is a lack of tongue abrasion, causing a keratin (protein) buildup called filiform papillae. They are conical in shape and get long, thus looking hairlike. Aside from a pureed diet, other things that lead to black hair tongue are: poor oral hygiene, smoking, oxidizing mouthwashes, oral antibiotics and excessive coffee/tea drinking. It most commonly happens in older people. The condition improves with brushing the tongue.
For this 50 year old man, the condition improved after 20 days.