The Washington Post had a good article on Black N's. It's been out there a while but I just stumbled across it while looking at a page on Robert Heinlein's Naval Academy career at the RobertAHeinlein.com site. There are probably not a lot of Alums out there with a Web site bearing their name.

Over the years many of us have probably had reason to consult the "Blue Book", which is the informal name for the Naval Register or lineal list. The Naval Register lists every officer in the Navy and his rank relative to every other officer. There are actually three Blue Books: NAVPERS 15018, Register of Commissioned and Warrant Officers of the Active Duty List; NAVPERS 15009, Register of Commissioned and Warrant Officers of the United States Naval Reserve; and
NAVPERS 15939, Register of Retired Commissioned and Warrant Officers, Regular and Reserve, of the USN. The Navy has now posted these lists as PDF files online, where they can be viewed in alphabetical, lineal number or designator order. Pretty cool and very useful for many purposes. This is part of the online version of the Bureau of Personnel (BUPERS) CD, a document full of of staggering ugliness, bad Web design and really useful information.