Ryan Moore bagged the only try and points for the under 16s in their game, although the Blues pack performed well in the second half and prevented the Ospreys from adding to their first half haul.
In the end, however, it was not enough, despite the Ospreys losing replacement wing Brett Pritchard with a broken ankle.
Jason Jones, Rhys Roberts and Rory Pitman crossed for tries for the Ospreys with Daniel Biggar, in his second season at this level, adding one conversion.
Last season's under 16 wing James Egan was amongst the try scorers at Sardis Road - Huw Dowden, Gareth Jones and Luke Ford put the Blues on their way before Egan made sure.
James Thomas added two conversions and a penalty for good measure whilst for the Dragons there were tries from wing Anthony Brown and replacement scrum half James Leadbeater.
A crowd of more than 400 saw the game - amongst them WRU manager Steve Lewis and Blues senior coach Dai Young. "The game was played at a high intensity and it was a great advert for regional rugby" said Blues regional performance manager Gafyn Cooper.
In the other under 18 game, the Ospreys were 31-26 winners against the Scarlets with John Jones, Lloyd Elliott and Ton Williams (2) going over for tries for the Ospreys. Lee Halfpenny kicked one penalty and Richard Cunniffe adding two more and a conversion.