From
http://www.philm.demon.co.uk/Miscellaneous/Vocabulary.html
Blue-Booking: A term originated by Aaron Allston (or at least his playing group) for a role-playing technique in which the actions of individual characters, especially out of combat and away from the main character group, are described in writing rather than speech. If this is done using a school exercise book or similar, a permanent log of the character's fictional life is thus created. Blue-booking allows for character development and minor "solo" plot activity without distracting the GM unduly from the main, group-based, plot. It evolved from the note-passing common in many playing groups as a means of dealing with individual character actions of which the rest of the PCs are unaware.