I just saw a youtube video about welding where they showed how a "black oxide" coating is applied to bare steel. Looks fairly simple. You buy a powder (phosphate?) in a bag and dissolve in hot water. You first degrease the part well in another hot water solution, dip in the oxide solution ~3 min, brushing the surfaces helps, wash in water, and dry. They said the oxide is a chemical conversion which doesn't change dimensions but adds microscopic pores. You then coat w/ oil and it gets absorbed to protect from rust. Seems that is what they do on "impact sockets" and similar tools. From what I know of physics, the black comes from the microscopic pores acting as "black-body cavities". Physicists would use stacked razor blade ends similarly to absorb laser beams.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans