Black boxes were devices which, when attached to home phones, allowed all incoming calls to be received without charge to the caller.
The black box placed a resistor in series with the line, so that the off-hook voltage was closer to -36V: just enough to stop the ringing, but not enough to trigger billing.
Black boxes were commonly built by phone phreaks during the 1960s to 1980s (and in some places like Eastern Europe, well into the 2000s) in order to provide callers with free telephone calls.
Another use of black boxes was in the incoming modems of computers running bulletin board systems that were popular back in the 1980s and early 90s.
The infinity transmitter, an eavesdropping device which in its original design relied on an audio path to the target line remaining open before a call was answered or after it was hung up by the recipient, was similarly affected by the demise of mechanical switching.