The Dragons looked set to earn a draw despite playing for almost 50 minutes with a man down, but Cardiff Blues denied them right at the end.
Gareth Anscombe's last-gasp penalty snatched a 19-16 victory for Cardiff Blues over the 14-man Dragons and Ulster gained derby revenge over Munster in the Pro14 on Friday.
The Dragons were a man down for almost 50 minutes after Lloyd Fairbrother was sent off for elbowing Dillon Lewis but looked set to escape with a draw at Cardiff Arms Park.
Josh Lewis' penalty and drop goal gave the visitors a 6-3 half-time advantage, but the Blues led 16-9 with seven minutes remaining courtesy of a Dan Fish try and 11 points from the boot of Anscombe.
The Dragons were not finished yet, though, and Jarryd Sage's converted try brought them level, only for Anscombe to settle it from the tee right at the end to halt Cardiff's four-match losing streak - leaving them three points behind third-place Connacht in Conference A.
Munster consigned Ulster to their heaviest defeat in the history of the competition in round five, but Dan McFarland's men responded to that 64-7 rout with a 19-12 success at Kingspan.
Ulster have now won four in a row and sit second in Conference B, with Rob Herring, Robert Baloucoune - yellow carded after only 11 seconds for taking out Darren Sweetnam in the air - and Dave Shanahan scoring their tries.
Two penalties apiece for JJ Hanrahan and Bill Johnston earned Munster a losing bonus point, which leaves them six adrift of Conference A leaders Glasgow Warriors - who have a game in hand - in second spot.
