Traditional blind mixes are based off of blind cryptographic signatures. These algorithms allow someone to sign something that is blinded, and the person who receives the blind signed item to unblind it. The signer does not know what the thing they signed will look like when it is unblinded, but they can verify that they have signed it. This can be used in several different ways to create a blind mix. https://en.wikipedia.org/wiki/Blind_signature This explains one implementation of blind mixing, although I think it is not a particularly interesting one. The most interesting implementations allow Alice to obtain blind tokens that can be used for paying anybody, without Bob first needing to get a 'deposit slip' to give to Alice. I don't know if this is what Zerocoin is based on, but blind signature schemes like this are what all of the traditional blind mixes are based on. https://en.wikipedia.org/wiki/Anonymous_Internet_banking