Earlier quoted context omitted.
Blue boxing predates 5ESS switches; 5ESS was notoriously the switch you couldn't blue-box.
Oh really? I always thought SS7 was the first switch to move to out of band. What's the last blueBox-able switch?
Esquire published "Secrets of the Little Blue Box" in the October 1971 issue of Esquire, based on the phone system MF design from the 1950s/1960s. That means blue boxing started at least 15 years before 5ESS, as tptacek pointed out.
The 1987 Phreakers Manual (http://fringe.davesource.com/Fringe/Hacking/Phreaking/Phreak... ) says "Blue boxing becomes harder as all Bell switching and transmission facilities go under to CCIS. Then to further complicate things, digital microwave, fiber optic, and satellite transmission are all coming to be digital and do not recognize 2600hz for the hang up signal. I predict that around 1990, blue boxes will be obsolete from all major cities."
The 2600 FAQ, Section C-07, says (the earliest date I found for this was August 9, 1993): "Because of ESS Blue boxing is impossible". This is incorrect. ... While the advent of ESS (and other electronic switches) has made the blue boxers task a bit more difficult, ESS is not the reason most of you are unable to blue box. The main culprit is the "forward audio mute" feature of CCIS (out of band signalling). ... So for the clever amongst you, you must somehow get yourself to the 1000's of trunks out there that still utilize MF signalling but bypass/disable the CCIS audio mute problem.
I don't know the switch models enough to say if it was exactly 5ESS, but everything suggests that that is the case.