well I wasnt there when he drank the stuff... so I cant be 100% sure. But from what I understand in the articles I have read about blue man, he put baking soda and salt in the water before ionizing it with silver. Aswell he didnt have a current limiting device so he was drinking water that was 500ppm plus.
My understanding is that the silver reacts with the chlorine in the salt, making silver chloride.