Ospreys fullback Chad Punchard and centre Deeke Morse crossed for first half tries as the Ospreys took control of the game to lead 15-10 at the interval.
Ian Smerdon bagged a try, conversion and penalty for the Blues North, but in the second half they were unable to close the gap far enough with replacement back rower Garrick Legge adding to their problems by being yellow carded.
Liam Powell scored a third try for the Ospreys, who were outstanding at the breakdown.
Smerdon kicked a second penalty whilst Tom Rees added a try for the Blues but it was all too little too late as the Ospreys won the title for a second successive year.
Gafyn Cooper, High Performance Manager for the Blues, said, "They were lethal at the breakdown and they turned over plenty of our ball and used it well. They were deserved winners."
Opposite number Dai Arthur said, "We are chuffed to have won it again, especially after losing twice to them during the pool games.
"It was another excellent game between two very good sides."
Ospreys: Tries: C Punchard, D Morse, L Powell. Conversions: C Banfield (2). Penalty: Banfield
Blues North: Tries: I Smerdon, T Rees. Conversion: Smerdon. Penalties: Smerdon (2)