A "Black Box" was also a device intended to prevent the (then
monopolistic) phone company from assessing charges to those calling you
long distance.
It functioned by putting an appropriate amount of
resistance in-line with your phone's wiring, and thereby preventing the
voltage shift from 36
volts to 10 volts that normally occurred when you picked up the phone. The phone company relied on this voltage shift to
signal that a call had been completed.
The Black Box had the distinction of being the cheapest of the
phreaker-age "boxes", requiring only a
switch, an
LED, and a
resistor.
(Use of a Black Box or Blue Box is also a good way to get arrested, as this writer discovered in 1985.)