It takes the North's winning run to five games although team manager Nigel Way has warned against complacency ahead of next week's showdown with the Scarlets at Narberth.
"We are obviously delighted with the win but a touch disappointed that we didn't pick up another bonus point. Things are going well, but next week's game against the Scarlets will be particularly tough."
Blues South put behind them their recent defeat at the Ospreys to pip the Scarlets 27-25 at Taffs Well.
Matt Roberts and David Young bagged first half tries for the home side who led 17-15 at the interval despite having Ceiron Driscoll yellow carded after 23 minutes.
Tom O'Brien crossed for a third team try after the break and Rhys Patchell added 12 points with the boot.
Gethin Hunter continued his good form for the Scarlets with two tries - that's six for the season - whilst Daniel Evans and Jordan Williams made sure of a bonus point for scoring four tries.
Williams ended the game with 10 points but finished on the losing side as the South held on in a frantic finish.
