A Blind Signature is a special type of digital signature
here the possessor of a private key signs data without knowing what the data is, and without later being able to verify that any particular piece of data was signed at a particular time.
This is achieved by first "blinding" the data by applying a so called "blinding factor", sending the data to the signer to get it signed, and later removing the blinding factor.
The technique is most often used in conjuction with so called cut and choose protocols, which allow the signer to probabilistically know that what he is signing is equivalent (though not equal to) some other data. An example is different well-formed notes/coins of the same value in digital cash schemes.
Blind Signatures is covered by US patent 4759063 by David Chaum, issued August 19, 1988.