These inexpensive carabiners have an
anodized aluminum surface that has been dyed.
Anodizing is used to provide a protective coating for aluminum and titanium. The process produces a controlled oxidized surface similar to bluing. Like the electroplating process, anodizing uses electrical current. Different colors can be produced in this process; black is the most common for firearms parts.
Other finishes that the firearm examiner may encounter include Parkerizing (phosphating coatings applied to steel), case hardening, and various paints.