Nothing beats the surge you get from coming on an old book folder that has a sig or two of blues in it.  That ammoniac smell takes you right back.  It is redolent of the successful completion of yet another job.

Blues, commonly called ozalids in UK, (Ozalid is actually a trademark in the USA) were the last proof you used to get before printing a book.  They were a cheap print made from the stripped-up negatives of the book (flats).  The flat was exposed to a sensitized paper, quickly “developed” and sent to the publisher as a proof that all was OK before plates were made from the same flat.  The paper was a creamy color and the type and illustrations appeared blue (the UK ozalid tended to be the opposite, a negative of the flat.  This was because of the right reading/wrong reading negative difference between the two sides of the Atlantic).

I always used to say that nobody with the word editor in their job title should ever be allowed to see a blue.  Editors regard a proof without any marks on it as a reproach — an indication that they were too sloppy to find the errors.  Of course by that stage in a book’s production, any correction would cost a fortune.  Thus it is probably only old production people who get weepy at the scent of a blue.