In this LH thread, AJP wrote:

While I’m at it, can anyone explain the US phrase “Police Blotter”. I understand the usage, but why a blotter? If you write on blotting paper the ink spreads until it’s indecipherable.

My response:

An excellent question, to which I have not been able to find an answer. The OED has:

4. A record of arrests and charges in a police office; a charge-sheet; also gen. a record-book or list. U.S.
1887 Harper’s Mag. Mar. 500/2 Every item of police duty, and of civil or criminal occurrence, is inscribed on the ‘blotter’.
1906 Atlantic Monthly Feb. 264 It was necessary..to examine the day-book or blotter in the chief clerk’s office [at the Patent Office].
1910 Washington Times 14 Dec. 1 Three more additions were made yesterday to the hospital blotters.
1926 J. Black You can’t Win xix. 299, I never put his name, which is my name, on a police blotter or a prison register while he was alive.
1965 ‘R. L. Pike’ (title) Police blotter.

But no explanation of the usage. I found a 1916 Columbus, Ohio: report on a survey of the city government with the following on p. 86:

3 DESK BLOTTER

No desk blotter is maintained. In order that the division may have a complete record of the activities of the uniformed force, a record to be known as the “desk blotter” should be maintained by the sergeant and the head of the detective bureau. The desk blotter should be a large bound bodk in which all movements of the force would be recorded chronologically. It should be the only record in the division known as a “blotter.” All other books should be known as “records.” The book should be ruled so as to have a marginal column on the left side of each page about two inches in width. The pages should be numbered. The entries in this book and the method of making them should be in accordance with specific rules. These rules should require that the blotter contains entries concerning the following:

The time the members arrived at the station and the time they left and the reason for leaving [etc.]

Very orderly and exact, but again, no reason for the name.

Any ideas?

There’s this, from dictionary.com (under “blotter"):

http://m.dictionary.com/etymology/blotter?linkId=8uxrdf

To paraphrase, with a few thoughts of my own thrown in in (), dictionary.com notes that “blotter” first meant (c1590) a sheet used to dry wet spots (from “blot").  By c1600 it had a sense of “bad writer” (presumably, based on the idea that one who needed to use the blotter frequently was likely to be unskilled.).  By 1670, it had a sense of “day book”.  By the early 19th century, it had a sense of a rough draft, a scrap book, a note book, or a draft account book, etc.  (Perhaps based on the day book sense, and/or perhaps based on the idea, jocular or serious, that such informal or rough drafts of works would require extensive blotting.).  The police blotter sense presumably grew out of the “draft account book” sense.

Along Svinyard’s lines, the OED blotter entry refers to synonymous waste-book:

ˈwaste-book, n.

Book-keeping.

A rough account-book (now little used in ordinary business) in which entries are made of all transactions (purchases, sales, receipts, payments, etc.) at the time of their occurrence, to be ‘posted’ afterwards into the more formal books of the set.

First citation is 1613

Speculation:  The large ledger books used for such purposes would be, when opened, comparable in size to a desk blotter, and if entries were frequent would probably wind up more-or-less permanently sitting open on the desk, occupying much the same position as a desk blotter.  It’s easy to imagine facetious references to it as a “blotter”, due to the foregoing, evolving into the standard term for it.

I found this on google books:

http://books.google.com/books?id=AhvoY4qJPTAC&pg=RA3-PA141&dq=Blotter+desk&lr=&as_drrb_is=b&as_minm_is=0&as_miny_is=1800&as_maxm_is=0&as_maxy_is=1900&num=20&as_brr=0&output=html_text&cd=47

A police officer specifically refers to a large book that is used by the police force as “the desk blotter” and also as “the blotter.” No explanation for why the term “desk blotter” was used is given, but this seems to lend some support to Dr. T’s idea that “blotter” (police or otherwise) may have come from “desk blotter” (the literal ones, that is).  It isn’t conclusive, of course, but it’s interesting.

Yes, I like Doc T’s idea.

Blotter doesn’t necessarily refer to a book filled with absorbent paper.  OED links blot possibly to plot, which is more in line with the sense blotter under discussion here - “plot” as in plotting a graph.  As svinyard noted above, an earlier usage referred to the book in which accounts were first recorded before being entered in the Journal then posted to the Ledger. There are several antedates in google books to the OED entry for blotter meaning book of records, which I believe is the sense under discussion.  This one from the “Reports from Commissioners, Revenue Arising in Ireland, Scotland &C; 16th Report, Revenue of Stamps Ireland” 1828 on page 76 refers to “the Four Courts Office, where the Law Fund duties are sold”, implying a pre-existing reference to legal matters in Great Britain at least.

The blot thickens.

Dr. Techie - 11 January 2013 12:33 PM

Speculation:  The large ledger books used for such purposes would be, when opened, comparable in size to a desk blotter, and if entries were frequent would probably wind up more-or-less permanently sitting open on the desk, occupying much the same position as a desk blotter.  It’s easy to imagine facetious references to it as a “blotter”, due to the foregoing, evolving into the standard term for it.

I speculate with Dr. Techie.

When I used a blotter, many important notes were made on it which caused the changing of the blotter to be a tedious task.

From ”The Encyclopaedia of Ephemera” by Maurice Rickards, Michael Twyman:

In America, the use of the word ‘blotter’ in the present sense dates only from about the middle of the nineteenth century.  Prior to that date, as also in Britain, the word was used to refer to a rough scribbling book or record book, most commonly in America the police station daybook.

I speculate with Dr. Techie.

When I used a blotter, many important notes were made on it which caused the changing of the blotter to be a tedious task.

I used to do the same and it is easy to see how this practice could have given it’s name to more formal ledgers used for the same purpose.