Mike
Mine must be old. Birchwood Casey AlumaBlack. Looks like they call it
Aluminum Black now. But it works quite good.
Try this,
Aluminum Black® Metal Finish http://www.birchwoodcasey.com/sport/index.html
The room temperature chemical used by gunsmiths and industry to blacken aluminum parts. Restores scratched and marred areas quickly. Fast-acting liquid is easy to apply with no dimensional change. Color will vary from deep gray to black depending on alloy. ALUMINUM BLACK Metal Finish is also excellent for blackening name plates, plaques, trophies and other engraved items.