Black pipe fittings (black malleable iron fittings) are used to make connections to black iron pipes. The primary purpose of black steel pipe is to carry natural gas into homes and businesses.
Black steel (Black iron) fittings are uncoated and since it has a dark color surface that is formed by iron oxide during the manufacturing process, it is called black steel pipe. Black steel fittings are manufactured as seamless which makes it a better type for gas transportation and fire sprinkler systems due to its high heat resistance and for water supply lines due to its resistance to water damage.
Black steel pipe and fittings are not suitable for water transport. Black steel pipes tend to corrode in water and mineral of the pipe will dissolve into the water and clog the line as well.