I've cured my parts overnight in boiled linseed oilicon - the traditional method according to Larry Mr. Potterfield, wiping off all the excess then hanging them up to dry overnight. I wonder if a different curing oil would make it darker? e.g. when I get back newly parked stuff, a liberal coat of RIG grease, rubbing it in good then wiping off the excess, can make manganese phosphate turn almost jet black.