The Dragons under 18s failed to prevent the Blues from moving a step closer to the WRU Age Grade Championship final, going down 33-14 at the Welfare Ground, Newbridge.
Despite having won their previous two games against Gogledd Cymru and the Ospreys, the region lost out to the table-topping Blues who also bagged a bonus point for good measure.
With the Scarlets defeating the Ospreys, also with a bonus, the Dragons are now almost certain to miss out on a place in the final later in the season.
But once again, there were signs that the side is making significant progress with team manager Billy Vyvian pleased with the performance.
"It has been a topsy turvy season in terms of results, but the aim is to help our young players develop and I believe we are developing nicely.
"We may have lost three of our five games, but the players have come on in leaps and bounds since the first game of the season. It's very encouraging."
Having swept all before them so far this term, the Blues completed a fourth straight victory in the competition despite, at one stage in the first half, trailing 11-3.
Owen Sheppeard, Jordan Sienawski, Alwyn Lee, Sam Campbell and Harry Robinson all touched down for tries with Joseff Griffin kicking a conversion and two penalties for good measure.
Leading scorer Matthew O’Brien crossed for the Dragons’ only try and added three penalties, but the visitors were too strong and maintained their unbeaten run at the half way stage of their season
Meanwhile having already beaten their regional neighbours this season, the Scarlets completed the double over the Ospreys thanks to tries from Iolo Evans, Rheon James, Luke Treharne and George North. James Russell added four conversions and as many penalties to secure a 40-27 victory.
Bowdy Davies, Ashton Evans and Scott Kelly crossed for the visitors at a wet and windy Drovers Field with Owen Williams kicking a conversion and penalty and Cameron Clement adding two conversions and a penalty to boot.
