Yes, black salt can be used as a substitute for regular salt but not in all recipes. Black salt has a distinctive flavor derived from its high sulfur and mineral content than regular salt.  Black salt also has less saltiness than regular salt, so when using black salt instead of regular salt, you may want to adjust the amount of other seasonings you use to balance the flavors of your dish.  Black Salt is coarsely ground, so it may not dissolve completely like regular salt.