hello i have gone through your query.A black tongue discoloration is most commonly caused by organisms such as yeast and bacteria growing on the surface of the tongue. These bacteria and yeast produce blood cell pigments called porphyrins which give rise to the black color characteristically seen. The overgrowth of bacteria can also cause cells of the tongue to be shed more slowly, causing the tongue to take on a hairy appearance, known as a black hairy tongue. maintainig a good oral hygiene is the best way to treat black hairy tongue. Gently brush your teeth twice a day with a soft toothbrush. Also, brush your tongue. You can use a tongue scraper to make sure you re thoroughly cleaning the area.If you smoke, quit.having soda wont cause this. antibiotics or an antifungal drug can be used to get rid of the bacteria or yeast. Topical medications, such as tretinoin (Retin-A) may help. hope this ans helps Regards Dr. shesh