If you search you'll find lots of "recipes". I used Birchwood Casey Rust and Blue remover but have also used white vinegar to remove bluing. It will also remove the "case hardening" finish the Italians use so be careful. Once all the bluing is removed if you want a stained metal effect you can re-blue or brown and use extra fine steel wool to get the effect you want.
I do recommend searching on this forum to find different techniques and the effects they produce.