A precinct's daily written record of arrests. In the days before ballpoint pens, much less computers, police station house records were kept with ink pens, the writing from which had to be blotted to keep the ink from smearing. Sheets of paper might come and go, but the blotter's indelible imprint remained as a metaphorical reminder of the name of the person who was arrested. That's why many old gangster movies has lines about the cops checking a bad guy's record on the police blotter.