Hi Captain!
Something I tried a long number of years ago on silver (to artificially "oxidize" it), is Liver of Sulfur (Potassium Sulfide). I have not tried it with iron (yet), but since it is chemical similar to Arsenic sulfide and leaves a black "stained" surface, it just might do the trick.
It is more readily available than Arsenic sulfide, and safer to work with too (but use plenty of ventilation), it just may be a good substitute. Buy it in dry form, mix about one to two teaspoons to one pint of hot water, and use within 48 hours. It will lose strength over time (that is the rotten eggs you smell as the sulfur leaves).
Let me know if you need more info. I would be interested in the results of any experiments made with this sulfide solution. I may work better in mild acid solution like lime juice...
Best regards,
BSMStar