The Black Box was a particular kind of device built by phone phreaks during the 1960s and 1970s (as distinguished from blue boxes and red boxes) in order to defeat long distance phone call toll charges, and specifically to block the supervision signal sent by the receiving telephone handset when the call was answered at the receiving end of the call.
In other words, the box fooled the phone company into thinking no one had answered at the receiving end, and therefore billing was never started on the call. Participating in a "black box" call meant talking between rings, since the phone co. would continue to send rings (AC current) through the line which was heard as a loud buzzing noise. Few people build or use "Black Boxes" anymore, given that the cost of long-distance telephone calls has come down dramatically since the 1970s.
See also: black box
