Kala Namak or Black Salt is a special type of Indian mineral salt with a very distinctive sulfurous mineral taste. It is indispensable in many chaats, chutneys, raitas.
Also known as Sanchal, Kala namak, is usually more of a grey-ish pink colour (when finely powdered), which will appear black when moistened, say when sprinkled on a raita or a liquid (like buttermilk) to lend its characteristic flavour.
Kala Namak 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 used in Jammu to cure goiters. This salt is also used to treat hysteria, and for making toothpastes by combining it with other mineral and plant ingredients. (Source: Wikipedia. Link to Kala Namak)