Straight olive oil gives a nice dark brown on warm steel.
Johnson's floor wax give a nice black.
For blackening my knife handles I use the vinegar that I use to loosen the scale from the blade. Just let it soak overnight (a jar full to the top with a lid will keep the hndle submerged). The penetration is generally good and the piece can be sanded (A LITTLE
). Linseed oil brings out the shine and helps mask the vinegar smell. Johnson's wax is a good finish
