Black oxide is a material used as far as I know on
drill bits to allow some benefit to the process of drilling. Probably to increase tool life as a company that makes
drill bits would say (they'll say anything about the tool or its accesories increases its life).
Black oxide is an ionic compound of either Cupric oxide, CuO, or Mercurous oxide, Hg2O. I believe the black oxide that is put on drill bits is cupric oxide since putting mercurous oxide would endanger the user's health.