Rust blacking is basically an oxidation reaction, nitric acid is a very good oxidising agent and a metal salt helps with the reaction and can deposit a small amount of ions on the metal surface to give a better finish. I used a nitric acid and copper sulphate mix for doing shotgun barrels some years ago and it worked nicely. Another chemical I tried that gave excellent results was a very dilute solution of silver nitrate, this is expensive but may still be easy to obtain since it used to be used to develop photographs.