After an initial period where the Blues North had the momentum, the Scarlets, led by openside flanker Will Boyd were dominant throughout. Full back Jamie Evans was first on the score sheet with a try after quarter of an hour's play.
The Blues, who had finished the season in second position after the Ospreys, were not able to finish off their attacks and a turnover led to the Scarlets' second try when wing Josh Adams ran over on the left wing, taking the half time score to 10-0.
Jack Maynard increased the lead to 13-0 with a penalty after the restart and Blues North found themselves down to 14 men after flanker Brad Llewellyn was sin binned.
Centres Steffan Evans and Craig Woodall added to the impressive try fest, with Carmarthen's Maynard improving both scores.
Brecon lock Jonathan James came off the bench to score a consolation try for Blues North but it wasn't their night and after a resounding victory for Blues over Scarlets in the Under 18 final, it was the Scarlets' turn to celebrate.
Scarlets head coach Rob Walters said, "It was a great achievement just to get to the final. It is the culmination of nearly a year's hard work for the players and coaches.
"Although we lost our first two games in the competition, the new format with a semi-final stage, allowed us to blood more players at a high level and by the semi-final, we were confident that on our day, we could beat anyone.
"The performance this evening was very pleasing, the boys put together a great display of running rugby and I just hope they continue to progress further from here."