A few websites recommend adding a dye or food coloring to the salt. However, as someone who cooks pretty regularly, I know exactly what happens when you add liquid to salt. Best case scenario, it gets clumpy, and worst case, it dissolves. So you'll want to use something dry to color it instead. Here's my basic recipe for black salt:
2 parts sea salt (which is the same as kosher salt)
1 part scrapings from a cast iron skillet or pot OR
1 part fine ash from your fire pit OR
1 part finely ground black pepper
Depending on the density of your coloring ingredient, you may need to adjust the portions a little, but that's the basic method of making it. If you have a well-seasoned cast iron pot, you should be able to get a good amount of black scrapings out of the bottom of it -- if it seems too oily, use the ash or pepper instead.
Don't confuse this concoction, however, with the black salt used in Indian cuisine -- that item is actually a mineral salt which is a weird pinkish gray color and has a bit of a sulphuric taste to it.