The Blues, who have had a mixed season, made a late bid to pinch one of the two places with a 24-10 victory over the Dragons at Abercynon.
Elliot Jones, David Barry, Ioan Mortimer and Dan Evans crossed for tries for the Blues with Diggy Bird converting two.
On Sunday, the Ospreys will wind up their pool games with a home game against Gogledd Cymru whilst the two regions will meet at Under 16 level too. Both games are due to be played at Llandarcy.
