The following is a post by a CoinZappers member, Chuck Burks. I think there is validity to the post and I have tried a variation of the posted technique.
How to Remove the Blackening From Zapped Coins
Paraphrased from Roger Peartree’s excellent letter to CoinZappers
A gentle rub with a thin slurry of sodium bicarbonate will remove the blackening left on a coin after zapping with no problem. Evidently, it is simply some sort of carbon deposit on the coin since it is not very tenacious. It certainly is not bonded to the coin the way a patina or discoloration would be. A very fine-bristled brass brush used with detergent is also effective at removing blackening. The brush has .003 diameter bristles and is obtainable from McMaster-Carr. It is finer than the brushes available at hardware stores. Not only are the fine bristles softer but the bristles can reach into minute details of the coins which are skipped over by the heavier brass bristles and the bristles of a toothbrush. Both the brush and the sodium bicarbonate will mar a smooth surface if the brush is applied too vigorously. Smooth surfaces are not generally an issue on coins but a very gentle treatment is advisable on all coins.