Black tofu isn't actually made from soy nor is it derived from any plant, so don't think it's for a vegetarian because it's solidified pork blood and is found at most Asian supermarkets under that name.
It is used in a wide variety of East Asian dishes and often found in soups or side dishes. It can be found usually over the butcher's counter in one giant slab that is chopped up into bite-sized bits upon sale, they are sold fresh. If you haven't guessed, it's called black tofu because it's an extremely dark brown, and looks a whole lot like tofu.
* This is usually just called "pork blood." Doesn't Black Tofu sound more tasteful though?