I've just tried the bleach method and it works so well! Soak your silver coins in it for about 20 mins (keep an eye on them; they go black quick) then wash them in water, then polish. I used a metal polishing cloth from a hardware shop but I'm sure any method for shining would do. The black stays in the recesses and looks fantastic (more contrast).

Thank you for all your help. This was my final decision. I did nothing. I decided that I shouldn't try anything. I tried some of the things out on other coins, but nothing was what I wanted. But thank you guys. (Though this last post looks good, I think I might try it too on some other coins; you guys are all quite creative.)
Rmax Goodwin

I found the silver polishing cloths that have been treated with a polishing compound are excellent for matching petinas. They work very slowly, with lots of elbow grease, so you can slowly polish a coin up, but not too much. You can get them from any jeweler, but they're also available from Lynetta Welch: http://www.FabricManipulation.com

Liver of sulphur (very caustic and dangerous) will work perfectly for silver. You can determine how much blackening you want. On the other hand, I do not recommend this and any injury you may sustain from this is your own darn fault!
David