I want to sign a message while not being able to see the message.
Logically, there should be a way, I just don't know how this translates into cryptography.
Here is an illustration for what I mean:
A man writes a message on a piece of cardboard. He wraps the cardboard in carbon paper and brings it to another man.
That other man is the signer. He puts his signature on the carbon paper, which in turn also transmits it onto the original piece of cardboard. However, the signer is not able to see the message, since it is hidden in the carbon paper. (The fact that the paper is fragile is without loss of generality, one can image something more tamper-proof of course).
The first man then returns home where he unwraps the piece of cardboard, with the message which is now signed by the signer. But the signer does not know the message still.
There are use cases for this and I would be very interested how this could be reproduced in cryptography.
EDIT: Trying to clarify something, I am sorry my inability to be more clear:
The thing is that the message creator may want to pass the message to third parties that he does not trust. It must be possible for a third party to read the message and verify that the message was signed by the signer. But, assuming that the signer signs many messages every day, it should not be possible for the signer and the third party to cooperate in order to identify which message the signer signed WHEN.