So as you can see some food dye in salt won't achieve those things. Recipes for black salt vary and are often guarded, they don't just contain salt that has been dyed. Several other ingredients are used but most importantly it goes through a cycle of blessings and ritual. Without those blessings it is simply coloured salt with the properties the white salt started with. Our black salt never gets to fully empty as the end of the last batch starts the next so we have energy and blessings going back years at this point in our blend.
When made correctly it is so potent you only need a small sprinkle each use so if you are using 500g to get the results it's not very good black salt!
So if making your own remember it will take research, it will take time to create your blessings and ritual around its creation!
If buying ask what they use to turn the salt black, ideally you want to avoid food colouring and absolutely avoid quick light charcoal blocks (please remember it's dangerous to breathe in their dust!). Also ask an open question of is anything else done or added to the salt. Don't expect to get the recipe, you won't! However if they are cagey, won't tell you what makes it black and have no other practices around its creation its a good indicator to try somewhere else!
A staple in so many people's magickal tool kit, let's also come back to having the knowledge of why this tool is just so special to begin with.