black oxide

Idiomatic Any of several coatings for steels or copper; some are oxides (mainly iron(II,III) oxide), some are nitrites or nitrates, and some are selenides; all are black; the main purposes are the antireflective and cosmetic effects; the anticorrosion effects are small.

Used other than figuratively or idiomatically: any oxide that is black; often, iron(II,III) oxide.