The phrase
"blue slip" typically refers to a piece of paper used by the U.S. Senate to show approval or disapproval of a judicial nominee. The color of the slip is not important; it is just a term. If a senator from the nominee's home state approves of the nominee, they submit a
blue slip to show their support. However, if they disapprove, they can withhold the
blue slip as a way to block the nomination.
Full definition