Great thread, thanks for all who contributed.
I have black oxide on my stranded copper boat trailer wiring.
Results of my tests:
1. hydrogen peroxide (3%): no effect
2. household ammonia (with surfactant): no effect
3. oxalic acid (Y-10 gel for cleaning fiberglass): no effect
4. phosphoric acid (Permatex Rust Dissolver Gel): black disappeared in a few minutes leaving strands a rosy pink color.
Nice thoughts on the H202 and ammonium hydroxide, but they didn't touch my oxide. Phosphoric acid is the way to go for me, unless I'm replacing the wires.