Black salt (kala namak in Hindi), which is used in Indian cuisine, is not actually black, but more of a grayish red or pink. It contains sodium sulphite, which accounts for its slightly egg-like taste (some vegetarian converts use it to make scrambled tofu taste like eggs). It's an ingredient of chat masala, and is also sometimes used to flavour drinks. In Ayurvedic medicine, kala namak is used to help digestion. You should be able to find it at any Indian grocery.
Apparently, "black salt" is also an ingredient used in casting spells1; I have no idea whether this "black salt" is actually kala namak, and I wouldn't recommend substituting one for the other.