Kala namak is called “black salt,” though that name might be marginally erroneous. According to Koyuncu Salt, kala namak can appear black, but can also have a violet hue. When ground up, the salt loses all sense of blackness and actually becomes pink. Even so, color isn’t really the point here — it’s all about the flavor.
Koyuncu claims that the salt has a high sulfate and sulfide content, which could account for it bearing a flavor very similar to eggs. The salt is laden with umami, which is part of what provides the yolk taste.
As the Food Network notes, kala namak is a rock salt found in volcanic regions in the Himalayas, as the name “Himalayan black salt” implies. (It’s also known as Indian black salt, as it shows up in the northern part of that country as well.) The region of origin is noteworthy because the seasoning has greatly influenced the food of the area. With an egg flavor built right in, Elephantastic Vegan suggests using kala namak in tofu scrambles or chickpea omelets. It can supplement other salts, or be used entirely on its own to increase and enhance the egg taste. If you’re looking to try it, you may need to seek out an Indian market or order it through an international vendor, since it’s not as likely to appear on standard grocery store shelves as regular table salt.