316L steel, or surgical steel as it is also called, is the material in which most of all piercing jewelry is made. It contains about 17% chromium, 12% nickel and 2.2% molybdenum, but it is so tightly bound in the metal that less than 0.1 millionth of a gram of nickel is emitted per cm2 and week. It is so small that 316L complies with EU directives for products that come into close and long-term contact with the skin. The limit value is 0.5 millionths of a gram / cm2 / week. "Ordinary" nickel allergy sufferers can usually wear 316L surgical steel, but if you are very sensitive or worried, you should choose titanium instead.
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.