Redwood Foods versions of these are sold in Holland and Barrett, Asda and wholefoods shops. Gravy granules. Just because you're feeding a vegetarian or vegan doesn't mean they have to have 'vegetarian' gravy (you know, the thin watery stuff that looks like a dissolved vegetable stock cube).
If your tongue is stained black or dark brown it may be caused by medications and certain foods/drinks. Drinking several cups of coffee or tea per day may stain the tongue; some people who supplement with green drinks containing chlorophyll may notice a very green tongue.
Also, mouthwashes that contain witch hazel, menthol or other astringents can cause black tongue. The oral use of medications that contain bismuth can cause black tongue. Heavy tobacco usage can cause black tongue, too. These causes change the chemistry in the tongue, which can result in substances that give the tongue a black appearance.