Ketosis?
Hey there! Just to let you know, black hairy tongue is harmless. It is caused by too much bacteria or yeast growth in the mouth. The color comes from the bacteria on the tiny round projections on your tongue (papillae) that are dyed by pigments in food, drinks, or the bacteria/yeast itself. It doesn't have to be black either- it can be brown, green, yellow, etc! You are more likely to develop black tongue if you have poor oral hygiene, smoke, drink lots of tea/coffee, use antibiotics, use medications with bismuth (pepto-bismol), regular peroxide containing mouthwash, or radiation therapy. If it doesn't resolve with brushing, then your provider will probably order an antibiotic or antifungal medication. Just letting you know. I am a RN

Black Hairy tounge is a bacteria condition in your mouth and has nothing to do with ketosis. Ketosis does however makes your breath, sweat, and pee stink as that is how the fat excapes from your bady as it is burned.
http://www.mayoclinic.com/health/black-hairy-tongue/DS01134
http://www.mayoclinic.com/health/black-hairy-tongue/DS01134