Black steel pipe is made of steel that has not been galvanized. Its name comes from the scaly, dark-colored iron oxide coating on its surface. It's used in applications that don't require galvanized steel.
Black steel pipe(Uncoated steel pipe) is called "black" because of the dark-colored iron-oxide scale formed on its surface; usually used for low-pressure hot-water heating pipes. Its available in two schedules (schedule 40 and schedule 80). The difference between the two schedules is the wall width of the pipe. Schedule 80 black steel pipe is thicker than schedule 40. In many jurisdictions schedule 80 is required for condensate line due to the acid and impurities. I would strongly recommend it over schedule 40.
When steel pipe is forged, a black oxide scale forms on its surface to give it the finish which is seen on this type of pipe. Because steel is subject to rust and corrosion, the factory also coats it with protective oil. Those black steel are used for manufacturing pipe and tube, which will not rust for a long time and requires very little maintenance. It is sold in standard 21-foot lengths TBE. It is widely applicable for ordinary uses in water, gas, air and steam, black steel pipes and tube are utilized for gas distribution inside and outside the house and for hot water circulation in boiler systems. It is also used for line pipes in oil and petroleum industries, for water wells and for water, gas and sewage purposes.
It is not used for potable water or for drain waste or vent lines.