Schnorr signatures let a signer prove they signed a message using their secret key. Blind signatures take this a step further -- you can sign a message without ever knowing the contents of what you are signing.
A user disguises their message, has the server sign the disguised message, then unblinds the result -- producing a valid signature that the server can't link back to the message it signed.
This is powerful for privacy: a server can act as an authority while remaining blind to the contents of the data it is authorizing.