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.
Blackline is a biocompatible PVD coating applied to titanium or steel with no corrupting effect on the metal.
They are not "colored" in the classic sense, it is 316L Steel or Titanium that is covered with a thin plasma film (PVD coating). The thin film is very strong and does not fade. The technology has been used in the medical industry since the late 80's and is used, among other things, for pacemakers and orthopedic implants to provide an extra durable surface.