Kala namak refers to Indian black salt. Black salt is a special type of Indian mineral salt. It is actually pinkish grey rather than black and has a very distinctiive sulfurous mineral taste, like hard boiled egg yolks.

This specialty salt is used in Indian cuisine as a condiment and is added to chaats, chutneys, raitas and many other savory Indian snacks.
Other definition:
Kala namak is a Hindi term for black salt
Other definition:
Kala namak is a Hindi term for black salt