Blind signatures let you sign a piece of data without learning what data you signed. Physical blind signatures are sometimes used the context of voting. You write your vote on a piece of paper, cover it with carbon paper, and have a voting authenticator sign the carbon paper without seeing who you voted for underneath. The carbon paper transfers their signature to your actual voting paper, which authenticates it, and then you can put your vote in the ballot box. By this method the person who authenticates your vote doesn't learn who you actually voted for. In bitcoin, digital blind signatures were just demonstrated using schnorr signatures, so now we can use those in bitcoin applications. Yay!