Cause of Black Tongue?

Following are the herbs, supplements and medicines that I take: Vitamin B-12, ginseng, fish oil, garlic, vitamin E, vitamin C, multi vitamin, caltrate plus vitamin D, synthroid, green tea.
Something is causing me to have a black tongue.

Bartram's Encyclopedia of Herbal Medicine (available at Richters), states that a black, hairy tongue may be caused by a bacterium, which may result from antibiotic therapy, smoking or poor dental hygiene. The only other cause that I know is from bismuth subsalicylate (for example, in Pepto Bismol).