First off, learn to do the signature vanish, which would be the root of this trick,

The Effect:
You ask a spectator for a coin and ask them to sign it, you say, "ok you see your signature? I'm going to make that vanish!!" Then using the same sleights from the root article, you make the coin turn black!!!

The Trick:
Ok, before the performance, make sure that you have a coin colored COMPLETELY BLACK with a sharpie, ok now, have the coin slit palmed to where you can slide the signed coin, underneath it, now, drop the black coin into your fingers and show them the coin, say something like, "AH!! That's not supposed to happen!!" It's a joke and it might help if you practice the slit palm before performing because you might drop both.