Kristian Baller gave the Blues the lead inside 12 minutes with a penalty from 35 metres, but after some slack defending around the base of a ruck deep inside their own half, the Blues fell behind to a try from Jon Jones which was converted by wing Leigh Halfpenny.
The Blues replied with a try from scrum half Tom Isaacs which Baller improved. After the interval the Blues started to make things happen with instrumental outside half Luke Ford.
Ford, son of former Wales wing Steve, helped to pin the Ospreys back in their own 22 whilst centre Tom Cooper provided further confirmation of his quality.
Tries for right wing James Egan and No.8 Matthew Larsen extended the lead and although the Ospreys threatened a comeback of sorts, it was the Blues who finished the stronger to help them carry off the coveted title.
Regional High Perfomance manager Gafyn Cooper said, "We are delighted to have a team of players who want to play for each other, many of whom will be involved in the Wales U19 and U20 squads for the upcoming internationals.
"They have been terrific all season and to see them play as well as this in the final is great. Credit also goes to our excellent team of coaches across our age grade sides. I hope things continue in the same vein."
Blues: Tries: Isaacs, Egan, Larsen. Conversions: Baller (2). Penalty: Baller
Ospreys: Try: Jones. Penalty: Halfpenny