Although kala namak salt is pink in color, it's often called "black salt" because the whole crystals are black. The salt takes on a pinkish hue when ground. It should NOT be confused with black lava salt, which is an entirely different ingredient.
Kala namak has a pungent, sulfuric aroma. Used in small amounts, it smells very much like eggs and makes an excellent addition to egg-substitute dishes like tofu egg salad and tofu scramble.
You can find kala namak at Indian groceries or gourmet/specialty foods stores.