The Collaborative International Dictionary of English v.0.48:

Blotter \Blot"ter\ (bl[o^]t"t[~e]r), n.
   1. One who, or that which, blots; esp. a device for absorbing
      superfluous ink.
      [1913 Webster]

   2. (Com.) A wastebook, in which entries of transactions are
      made as they take place.
      [1913 Webster]

WordNet (r) 3.0 (2006):

blotter
    n 1: absorbent paper used to dry ink [syn: blotting paper,
         blotter]
    2: the daily written record of events (as arrests) in a police
       station [syn: blotter, day book, police blotter, rap
       sheet, charge sheet]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "blotter":
   Domesday Book, absorbency, absorbent, absorption, account book,
   address book, adsorbent, adsorption, adversaria, album, annual,
   appointment calendar, appointment schedule, assimilation,
   blankbook, bloat, blotting, blotting paper, boozehound, boozer,
   calendar, cashbook, catalog, chemisorption, chemosorption,
   classified catalog, commonplace book, court calendar, daybook,
   desk calendar, diary, digestion, diptych, docket, drunk,
   endosmosis, engagement book, engrossment, exosmosis, guzzler,
   inebriate, infiltration, journal, ledger, log, logbook,
   loose-leaf notebook, lush, memo book, memorandum book, memory book,
   notebook, osmosis, pad, percolation, petty cashbook,
   pocket notebook, pocketbook, police blotter, scrapbook,
   scratch pad, seepage, sorption, spiral notebook, sponge, sponging,
   table, tablet, triptych, workbook, writing tablet, yearbook



Bouvier's Law Dictionary, Revised 6th Ed (1856):

BLOTTER, mer. law. A book among merchants, in which entries of sales, &c.
are first made.
     2. This book, containing the original entries, is received in evidence,
when supported by the oaths or affirmations of those who keep it. See
Original entry.