The North, who enjoyed an outstanding season last time around, ran out 34-19 winners against the Scarlets at Merthyr, whilst the Ospreys recovered a 10-0 half time deficit to defeat the Dragons 21-17 at Llandarcy.
Owain Rees, Elliott Parker, Darren Harris, Jack Dando, captain Callum Thomas and Shaun Webber crossed for the Blues' tries with outside half Lewis Williams converting two of the half dozen.
Gethin Hunter and Curtis Morris scored the Scarlets' tries with Luke Treharne weighing in with seven points from a brace of conversions and a penalty.
North age grade manager Gafyn Cooper, said; "When you consider that we struggled in the first season or two, I think the coaches and the players can be proud of this continued success.
"We did well last season and this was another excellent start from the boys."
Meanwhile at Llandarcy, it was the Dragons who held the upper hand for much of the game after striking first with Bradley Hill crossing for a first half try and Thomas Haynes adding a conversion and penalty.
Hill bagged a second try late in the game - Michael Davies converting - but by that time, the Ospreys had secured an opening night victory courtesy of tries from Alex Evans, flanker Will Thomas and Nathan Brooks.
Tom Carpenter made sure of victory with three conversions and team manager Graham Court was delighted with the victory.
He said, "They are an impressive side and were deserving of their lead. However, all credit to our lads, they battled back and made sure that it was a winning start to the season."