late 14c., "a spot or stain of ink;" also "a moral stain or blemish, a disgrace, a sin;" of uncertain origin. Perhaps from Old Norse blettr "blot, stain," or from Old French blot, variant of bloc "block." The Middle English Compendium compares, hesitantly, Old French blo(s)tre, variant of blestre "a boil." From 1570s as "any black or dark patch."
1590s, "thing for drying wet spots," agent noun from blot (v.). Meaning "bad writer" is from c. 1600. Sense of "day book" is from 1670s, and the word was applied by 1810 to rough drafts, scrap books, notebooks, and draft account books. Hence the police jargon sense "arrest record sheet," recorded from 1887.
The Waste-Book, or Blotter, is nothing different from the Journal, only from the circumstance that it is used in moments of haste during the business of the day, when it is not practicable to observe that precision, neatness, and order, which we wish to appear on our Journal, which is nothing more nor less than a better finished copy of the Blotter itself .... [Lyman Preston, "Preston's Treatise on Book-Keeping," New York, 1835]
"blot out, cause to disappear, remove all traces of, wipe out," c. 1600, from Latin obliteratus, past participle of obliterare "cause to disappear, blot out (a writing), erase, efface," figuratively "cause to be forgotten, blot out a remembrance," from ob "against" (see ob-) + littera (also litera) "letter, script" (see letter (n.)). The verb was abstracted from the phrase literas scribere "write across letters, strike out letters." Related: Obliterated; obliterating.
"act of obliterating or effacing, a blotting out or wearing out, fact of being obliterated, extinction," 1650s, from Late Latin obliterationem (nominative obliteratio), noun of action from past-participle stem of obliterare "cause to disappear, blot out (a writing)," figuratively "cause to be forgotten, blot out a remembrance" (see obliterate).
1580s, "a word or passage deleted;" c. 1600. "act of blotting out or erasing," from Latin deletionem (nominative deletio), noun of action from past-participle stem of delere "destroy, blot out, efface" (see delete).