When two people trade each other a CD the other has never heard before. Kind of like a blind date, meaning you don't know what you're getting cause it could be anything at all (except a lot less awkward than a blind date).
Keith and I did a Blind Trade the other day. Keith gave me "Kid A" by Radiohead (which I had never heard before), and I gave Keith "De-Loused in the Comatorium" By The Mars Volta (which was completely new to him). It was a good Blind Trade!