The Jargon File (version 4.4.7, 29 Dec 2003):

blue box


    n.

    1. obs. Once upon a time, before all-digital switches made it possible for
    the phone companies to move them out of band, one could actually hear the
    switching tones used to route long-distance calls. Early phreakers built
    devices called blue boxes that could reproduce these tones, which could be
    used to commandeer portions of the phone network. (This was not as hard as
    it may sound; one early phreak acquired the sobriquet ?Captain Crunch?
    after he proved that he could generate switching tones with a plastic
    whistle pulled out of a box of Captain Crunch cereal!) There were other
    colors of box with more specialized phreaking uses; red boxes, black boxes,
    silver boxes, etc. There were boxes of other colors as well, but the blue
    box was the original and archetype.

    2. n. An IBM machine, especially a large (non-PC) one.


The Free On-line Dictionary of Computing (30 December 2018):

Blue Box

    The complete implementation of the Mac OS
   run-time environment on the more modern Rhapsody operating
   system.  Blue Box is not an emulation layer; at any given
   time it will be based on the same source code and ROM image as
   the current version of Mac OS and will thus incorporate future
   Mac OS improvements.

   (1997-10-15)