A blind signature lets somebody vouch for a document without being able to read it.
The picture to hold is a sealed envelope with a sheet of carbon paper inside. You sign across the outside, and the signature comes through onto the sheet within. The signature is genuine. The signer never saw what they were signing.
The cryptographer David Chaum described it in 1982, at Berkeley, and it was the foundation of the first digital cash that actually worked: a bank could certify that a coin was valid without knowing which coin, or whose. In 1988, with Amos Fiat and Moni Naor, he added a further twist — a coin that can be spent offline and anonymously, but where spending the same coin twice makes the payer’s identity fall out of the mathematics on its own. Anonymity as a condition rather than a promise: you lose it exactly when you cheat.
It still ran into the wall this whole subject keeps hitting. Detecting a double payment afterwards is not the same as preventing one at the time. To be certain in the moment, somebody reachable still has to say: that one is already gone.