When I heat blue using a torch, my quenching oil of choice is olive oil. I cant tell the difference in finished color between using olive oil or brownells quenching oil

Larry Schuknecht of Dutchman Woodworks does a nice job of blacking using niter salts. Very black, smooth finish. Even he admits though, its not a very durable finish. I believe a lot of early American SxS guns had their furniture finished using the niter method. .