black box NES games

on digitpress i have heard some of the NES games called blackbox games. some of them were wrecking crew, mario bros, duckhunt, donkey kong, and gyromite. what is a blackbox game?
 
my guess is that the box is black with a picture in game on the front i just sold a gyromite complete and it was blackbox also i know mariobros and duckhunt is the same
 
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