i had good luck with boiled linseed oil and an old grill.
WARNING boiled linseed oil is highly combustible and flammable. leaving the oil on a rag will cause it to combust if not discarded properly. i soaked mine afterwards in a 5 gallon bucket of water and left it in there.
I took an old CD helmet and cut it down. then i sanded off all of the color and painted the boiled linseed oil on with a brush. then i put it on an old grill in the middle of the yard and walked away. when I came back it was a nice shiny black with a little red hugh here and there. a little crusty on the edges where the oil dripped but that is easily overlooked.
could also try some metal bluing like used on rifles