Thomas, who made such an impression for the senior Scarlets side earlier in the season, helped himself to a couple of tries with Owens adding a third. Gareth Jones replied with a try for the Blues and Christian Bala added a conversion and penalty for good measure. However, the table-toppers never recovered from a 13-3 interval disadvantage and must now wait until next month at least before confirming their place in the final.
In the other game, the Ospreys were easy 45-8 winners at the Dragons for whom this season has been very difficult. Leading 24-3 at the interval, the Ospreys completed the double over the Gwent region thanks to seven tries, and five conversions from Leigh Halfpenny.
James Pike began the rout with a try in the opening 10 minutes, which was followed up by further tries from wing Jason Howells, centre Liam Rice, and two each for Halfpenny and prop Phil Price.
The Dragons replied with a try from the hugely impressive replacement loose-head James Owen and a penalty from outside half David Langdon. Regional High Performance Manager for the Dragons, Ron Schottle said, "It's disappointing but I felt as though our forwards went well and perhaps deserved a bit more."