In the under 18 Championship, the Blues gave a wonderful performance at Stradey Park before going down 33-30 to the Scarlets.
Captain and openside Cory Hearn bagged the single try for the South in their gamne with the Scarlets, but it was the boot of Luke Fear that won the day - the excellent kicker added a conversion and a brace of penalties to make sure the South won the day.
Regional performance manager Gafyn Cooper said, "It was a good win for the South because the Scarlets are a good side. In the end we showed great commitment and I was delighted for the boys that they came through to win."
The Blues North were plucky in the face of some intense pressure from the table-topping Ospreys, who ran out 25-8 winners at Corus Rugby Club.
Sam Shelley crossed for a try for the North with Harry Alan adding a penalty. However, the Ospreys ran in two second half tries to help build on their 10-8 half time lead.
The under 18s, who defeated the Dragons last week, were beaten 33-30 by the Scarlets in a thrilling game at Stradey Park.
Matthew Edwards, the outstanding scrum half Rhys Dyer, James Toller and Tom Price helped themselves to tries with James Thomas adding two penalties and as many conversions. The Blues came away with two bonus point for scoring four tries and finishing within seven points of the Scarlets to leave them second in the table with the Ospreys up next at Pontypridd RFC.
However, the Scarlets had four tries of their own and full back Daniel Evans added 13 points to deny the Blues a second successive victory of the campaign.