I would not introduce bluing to a vessel. Paint would be the best solution as that is what the vessel manufacturers apply themselves. That said All pressure vessels that get blued are blued inside and out, but done by proffesionals that know how to start and stop the oxidation process. That is why they call it Black Oxide & Oil. You can't just do the Oxide part. Once blued properly the vessels are no more inclined to rust than a raw piece of steel.