Thanks for the report, dmurvihill!
Well, if you buy a Port from the Black Market, where do you think the other Port is going to come from? There is no Port pile in the game, so while you are correct that the trigger formally happens, it cannot have any effect, so the game doesn't show it at all.
Similar effects happen for example with Wild Hunt/Farmers' Market, which cannot accumulate VP tokens on their pile when they come from the Black Market deck. So Wild Hunt is still good for drawing and maybe gaining an Estate when the game is real close, but Farmers' Market is only good for the +buy, it will never give you $ or VP tokens.