Black salt is an unrefined mineral salt. It is actually a pearly pinkish gray rather than black.
Black salt is considered a cooling spice in ayurvedic medicine and is used as a laxative and digestive aid. It is also believed to relieve intestinal gas and heartburn. It is sometimes used by people with high blood pressure or on low-salt diets because it is thought to be lower in sodium and purportedly does not increase sodium content in the blood.
Black salt is appreciated by vegans in dishes that mimic the taste of eggs. It is used, for example, to spice tofu to mimic an egg salad.