I'm not sure where black salt comes from and I don't know what it's made from. I also don't know why it's called black salt because it's pink, but you do need it for this recipe.
I got my supply in Main Bazaar in the Pahar Ganj district of Delhi. Most people who have visited the city will know the area. It's a warren of narrow lanes lined with clothing shops, souvenir shops, cafes and cheap backpacker hotels and although you can take a taxi or rickshaw along most of the lanes, it's generally faster to walk. I was stocking up on spices from a shop I regularly go to just before flying out of India, and the proprietor suggested I try some black salt. He said it was good in salads, but I forgot that until recently. I began to use the spice in potato chat as I knew it was one of the ingredients after reading the label on a packet of chat masala in a supermarket in Chennai (I sometimes do that to get ideas for recipes).
A few weeks ago I was doing a big Indian style spread and instead of preparing the usual mixed raita, I decided to make a simple opinion raita and a separate tomato salad. I was wondering what to season the salad with when I remembered the shopkeeper's comments about using black salt. He was right. It is extremely good in salads. It combined with the tomatoes to produce a tangy sweetness with an elusive hint of strawberries.
You probably don't have to go to India for black salt. I guess it's available in Asian food stores or from specialised spice suppliers all over the world. It's worth checking out.
Chop the tomatoes and coriander and mix together, adding the black salt and ginger.