Never heard of the quenching in oil trick...but then, blackened hardware does not sound like it it is too far removed from ugly gnarly looking fasteners.
Is all your hardware rusted? Sure about that? Try cleaning before you right it off. I like 0000 steel wool soaked in wd40, then wipe off excess with rag. Super time consuming but I am always amazed how much nice plating is left on the crustiest of bikes. If you got solid brown, pitted rust, the plating is gone...if I am not gonna be bothered with replacement, wire wheel it, coat in wd40 and send it. Yeah, its gonna rust again, but no worse than when you found it.
And then there is more drastic and production oriented methods like re-plating, tumblers, home plating kits, etc....