My personal opinion is that the idea salt can rid one's house of evil spirits/black magic is just a superstition crafted by clever salt traders in ancient times to take advantage of superstitious folks and boost profits.
What do you think? How would a chemical influence a spirit?
A spirit is the energy of a person. The "breathe" of life and what keeps everything going. When you are around people, depending on how empathetic you, can physicall sense their spirit-their energy. It can be like a light breeze or it could be stuffy or closed in feeling. When you pass away, your energy still exist. It doesnt disapear because you have no body.
In nature (we are part of nature)
"Salt absorbs dirt, grime, and toxins and cleanses your skin's pores deeply.Salt's mineral content helps restore the protective barrier in skin and helps it hold hydration. ... The magnesium in sea salt may also help reduce water retention (i.e., bloating) in the body." -to the Wiki
Many put salt in the corners of their home because salt obsorbs literally bad toxins in the environment. It cleans the energy thereby cleans spirits energies. Its the same energy you experience when around a person. Energy doesnt change just because it has no body.
My mother put salt in corners ofnher home. My grandmother, I think, put salt under the front door, doormat. All to literally obsorb negative energy (spirit) from a person or cleans the spirit so there wont be upset in our home.
Many neopagans use water becauae its part of the earth and we live off the spiritbof the earth. Salt is a huge factor in seawater which is something we need to survive in general.
The supersition or unexplained common connections to things we do is basically biblical. Christians see a person using salt, wearing a upsidedown pentacle, naked, drawing something in the sand, and I kid you not, most christians would call him devil worshiping of some sort.
The usage and purpose of salt in religion isnt supersitious if you understand why our body needs certain things to live. For example, we can live without the bible. No one can love without salt. So it makes sense to correlate our physical survival with that of our spirit becauae they go together. Hence why you have Chakras and people meditate etc.
Its all about energy. Using external means to cleanse energy around you. In this case, energy of people (all nature): spirits.