They are made of high quality stainless steel. To keep your jewelery shining for so long, take the hoops out before showering or swimming.
Steel Blackline® is black jewelry, not "dyed" black in the classic sense, it is 316L Steel covered with a thin plasma film (PVD coating). The thin film is very strong and does not fade. The technology has been used since the late 80s and is used, among other things, for implants to provide an extra durable surface.