Black salt is used as a protective element. Sprinkled around your property, it  keeps your space safe from intruders or evildoers in any form. It is traditionally used to drive away evil, and can even be sprinkled in the footprints of someone who is bothering you, to make them go away.

Make Your Own Black Salt

You should cast a circle before you begin. You should also state your intention out loud before you begin. Should you purchase Black Salt, before using cast a simple circle, state your intentions and ask for blessings. Do not ingest Black Salt or give it to anyone to ingest.

Here's a basic recipe for black salt:

• 2 parts  dead sea salt
• 1 part scrapings from a cast iron skillet or pot or anvil dust
• 1 part fine ash from your ceremonial fire pit or smudge bowl or charcoal
• 1 part finely ground black pepper 

In addition to being a powerful ingredient in protection magic, black salt is used in some folk magic traditions for cursing, hexing, and binding. Obviously, if your beliefs prohibit you from doing this sort of working, don't do it - and feel free to just skip to the next section. However, if you're okay with magic of this nature, black salt can be a valuable tool.

The folks over at Black Witch Coven, ( https://blackwitchcoven.com/)  a compendium of all kinds of great hoodoo and conjure information, say

"To gain revenge on an enemy, sprinkle black salt into a doll baby or voodoo doll containing a personal concern from the enemy, such as a photograph, snip of hair or fingernail clippings. Black salt can be added to black magic mojos or bottle spells which are to be buried on the property of the victim or else hidden inside their home or car. But you have to make sure that no innocent person is harmed in any way."

Other uses for black salt in hexing and cursing include mixing it with other ingredients such as red peppers, grave yard dirt, or war water .

Black Salt for Protection Magic 
( https://www.learnreligions.com/use-of-black-salt-2562587)

As mentioned, black salt is primarily a protective magical tool. I like to sprinkle it around the perimeter of my property a few times a year to keep unpleasant people or things from crossing into my yard. You can also use it at work — tuck a small bagful under your desk to prevent annoying coworkers or the office bully from hanging around. If someone you dislike is leaving your home, wait until they're gone, and then follow along where they walked — toss some black salt into their footsteps to keep them from returning. Dress a black candle in oil and then roll it in black salt, and use it in spellwork for  for banishing negative energies or people.

If you've got leftover black salt after a working, depending on what you've used it for, it's something you may want to go ahead and get rid of. To dispose of black salt, if you've used it in a hexing or banishing, take it someplace far from your home and bury it, or throw it into a fire. If you've simply used it for a protective boundary, you can bury it on your own property or leave it to sink into the Earth.

Disposing of Black Salt

If you've used black salt in cursing or hexing, you're going to want to get rid of it eventually. After all, you don't need to keep it hanging around. There are a few easy ways to dispose of it. You can take it somewhere far from your home and bury it; many Hoodoo and Conjure practitioners recommend burying it near a crossroads or even a graveyard. You can also toss it into moving water, like a stream or river. Make sure the water really is moving, though — you don't want the salt just swirling around in one stagnant spot. Finally, consider disposal by fire. If you choose to use this method, however, be sure to take the ashes far away and bury them — don't use them for later magical applications.