Salt has been used for many purposes throughout history, both magic related and non. One of the most common uses for salt, is for protection and Black Salt has been known to be a stronger form of protection vs using regular salt, though either will do for whatever purpose it is needed.
Salt in general has many properties related to protection. Sprinkled in the windows, around the doors, used to cast circles, etc.
There are two different types of Black Salt.
Black Lava Salt which is commonly used when mixing sea salt and activated charcoal. You can use this to flush the body of toxins, or sprinkled in your home to absorb negative energies/toxins. You must sweep up the salt afterwards, to get rid of those toxins that it has picked up.
Black Himalayan Salt which is not actually black at all. It's usually a pink or purple color when whole, and then when ground up the colors get more pale, still keeping their pinkish or purple hue. Made by combining charcoal and various herbs, then heated and aged to get its popular fragrance. This salt is used in aiding in digestive problems, and things like flatulence.
These aren't the only two, of course. Just two of the most common types.
Black salt also has many different recipes and variations. One could use the burnt scrapings from the bottom of a cauldron or pot, I've heard of egg shells and also black pepper mixed in to create it.
I got most of my information from Witchipedia I believe. This is written in my BoS so I'll have to double-check my sources and link them once found.