How is Persian blue rock salt made?
Persian blue salt is formed by evaporating sea water, as are most if not all salt deposits. In the geosciences, we call salt deposits like this evaporites. Persian blue salt is sodium chloride (the mineral name is halite).
According to this description:

Blue salt is one of the rarest salts known to mankind, and is extracted from a salt mine in the northern province of Semnan in Persia.
The intriguing blue color occurs during the forming of the salt's crystalline structure, as intense pressure is exerted on the salt deposits. The individual crystals fracture the light in an unusual way and the resulting blue (which is caused by an optical illusion), becomes visible.