Originally Posted by DWalt

Most anything acidic will damage or remove bluing. As bluing is essentially a form of rust, anything which attacks rust (Iron Oxide) will damage it. Naval Jelly certainly will, and NJ is a sort of thickened Phosphoric Acid. I have used weak hydrochloric acid to remove bluing. Blood will definitely damage bluing. A long time ago I had a cop friend who used his revolver to bludgeon a BGs head, and got blood all over it. He had to send it back to S&W for a re-blue job. I also saw a Colt OP revolver used in a suicide that had lots of speckled bluing damage from the blood. I could have bought it very cheaply from the owner, but it creeped me out, and I passed.