After playing some more, it seems that what is happening is that I don't have enough coin to buy any of the cards from the Black Market, and the cards are being returned because of that. The cards should stay visible until I choose not to buy. This is because if the Black Market play was forced (Golem, Prince, Herald, Summon, etc.), the whole thing happens in an instant and I have to look in the log to see what happened.
In addition, there is no autoplay treasures button; instead, there is a "No treasures" button whose purpose is a complete mystery to me. Why would I click a button to not do something? The two buttons that should show up during Black Market are "Autoplay treasures" (which disappears/dims if there are no treasures to play) and "Put back cards".