I still have yet to find a store that still sells it. But Birchwood Casey Aluminum black is a wonderful thing when it comes to airsoft metal. It achieves a similar effect to cold blueing on steel. I just cleared the metal of shitty WE paint and applied that stuff with cotton balls, then simply used it a bunch.
Simply handling the guns will wear the stuff off from the metals surface, something akin to what you'll find with a lot of older handguns. Holsters do a good number on them too.
This was what it looked like right after the metal treatment. Admittedly, kinda wish I kept it this way. But the aged look is nice too!