A blotter was an absorbent pad used to catch spilled liquids.
In 2266, Leonard McCoy wondered aloud where James T. Kirk and Spock were, as they were late returning from a visit to Jojo Krako. Bela Okmyx suggested Krako would "probably send them back on a blotter." (i.e. liquefy them) (TOS: "A Piece of the Action")