If an opponent gets a card out of the black market deck and you normally have an autoplay option selected, the game uses that autoplay without there being anyway to turn the autoplay off during the game as the card isn't present in the supply.
eg the opponent got mountebank out the black market and I had watchtower and curse in my hand and there were no other cursers and I automatically discarded the curse rather than accepting and immediately trashing the curse and copper to run down the curse pile
eg the opponent got mountebank out the black market and I had watchtower and curse in my hand and there were no other cursers and I automatically discarded the curse rather than accepting and immediately trashing the curse and copper to run down the curse pile