Blues South, who sat out of the opening round of games, proved combative throughout a game ruined by the wet conditions, but in the end managed just a bonus point for finishing within seven points.
Having won their opening game 24-12 at the Scarlets, the North took the spoils thanks to first half tries from Owen Williams and Kieron Marmion and a conversion from Joseph Griffin.
A 29th minute try from John Lewis and a penalty from Diggy Bird kept the South in touch but it was not to be as last season's beaten finallists hung on for a crucial victory.
North Assistant Coach Sophie Bennett said, "It was a tight game with credit going to both sides for trying to play rugby in such difficult conditions. However, we are glad to have won for a second time this season; it's certainly a good start for us."
Meanwhile, the Blues U20s take on the Dragons at Glamorgan Wanderers this evening (7.15pm kick off). Last season's Wales U19 captain Sam Warburton starts on the bench having successfully recovered from a closed season operation on his knee. Brother Ben Warburton is also named as a replacement with the side led by Llantrisant's Jevon Groves.
Blues U20: N Rees; J Egan, F Edwards, H Allen, J Hannam; L Ford, L Evans; S Hobbs, H Dowden, S Andrews, M Al-Azzawi, J Groves (Capt), J Thomas, D Franks, K Dacey
Replacements: R Williams, P Palmer, J Porter, S Warburton, B Warburton, J Thomas, L Williams
Dragons U20: TBC