If you research "silver patina," you will find some very innovative ways of tarnishing (we call it toning) silver engaged by jewellery makers. I don't see why something won't happen to a salt print when exposed to some hard-boiled eggs. Practicality is an another matter. You can try the next time you make an egg-salad... I have tried Liver of Sulfur, another favorite of jewellery artists (which is also the basis of polysulfide toner) and so far the results have been analogous to the black salt. If you are suggesting use of bare H2S gas, synthesized somehow in-situ and exposing the print, I would be careful as it is toxic if over-exposed to - not something I would like to play around with. :Niranjan.