Black salt is a mixed preparation of salt and a few choice items that turn that salt from its usual white color to a black color. You can Google recipes for it, and there are also people who manufacture it, usually by somehow adding a pigment to the salt that turns it black. Some other names for Black salt is also Sal Negro (Spanish for Black salt) and Witches’ Salt. In this case the witches being referred to are not the contemporary followers of a neopagan religion, but rather the historical and folkloric “witch” a figure, sometimes supernatural in nature, who use supernatural powers to do harm to people and places in various ways.
As the names suggest, the primary use and purpose of black salt is to curse or perform harmful magic. A common secondary purpose is used to drive away people from your home. This is also a form of a curse, although a curse done to protect oneself and property from the person who is already causing harm. It is similar to hot foot powder in that sense, as hot footing someone is also a type of curse meant to drive someone away, whether that person is vexing neighbor, a bothersome coworker, or a conniving love rival. Ultimately both are curses, are not in anyway beneficial, benevolent, or really kind acts to be engaging in, however necessary they may be for one’s personal health, wealth and well being. Remember that the color most often used for cursing is Black. And the historical and folkloric image of witch (whose salt this is) is a supernatural agent of harm and evil. Black salt is used to curse, and sometimes that curse is used to drive away other evil beings, but the essence behind it is cursing and doing harm.
There seems to be a new fad of promoting black salt as something not as bad. I have read online various indications of use black salt for protection, by laying a circle of black salt around your home. This is a technique one might do with regular salt, as salt is protective, and purifying. Black salt, however, is the opposite of that. Laying a circle of black salt around your home would probably only curse you and start ruining any peace or prosperity in your home, instead of protecting you from harm.
Elsewhere I have read suggestions to use black salt with the dead and with ancestors. That concept is also ignorant usage of black salt. For one, a great number of methods and styles of venerating ones ancestors and conjuring up the dead indicate not to use salt. This applies to any kind of salt, not just regular salt. While the various metaphysical explanations behind this are multitude, it still comes down to one thing. Don’t do it. The ancestors and the dead don’t like salt. For the few methods that do use salt, one should keep in mind that it is unique to that culture and tradition, and the salt they use is going to be regular salt, not black salt.
I have also read suggestions of using black salt with gods, usually gods of war, death, the underworld. Again, I find this to be incorrect, although there could be an exception. The exception would be calling upon that specific god to bless the black salt, before you go out and use it to drive away evil or to curse. That is not the same as using blacks salt in devotion and veneration of the god or gods in mind
It is possible that in your personal work, you might find a spirit that wants to have black salt in it’s presence. If that is the case, then do so, but remember that it is unique to you, and unique to your practice, and in no way reflects traditional uses and folklore behind black salt and if you do write about it, you should make that clear to your readers.
I have even read people suggesting to bathe in black salt, although I suspect this might be a recommendation for a special kind of salt that is mined from the ground, which has a dark gray to black color to it. That would function as normal salt, not as the special mixture known as “black salt” which is salt mixed with things