Jake Randall crossed for a first half try for the Scarlets with leading scorer Jordan Williams on the mark with a conversion and two penalties as the hosts turned around 13-3 to the good.
A Gareth Thompson penalty was all the Blues South could muster in the first half, but they made life particularly uncomfortable for the Scarlets after the break with Thompson converting a try from wing Ben Harding.
The South are now a point clear of third placed Ospreys who have a game in hand.
It was a successful night all round for the Blues - the under 18s, who have already qualified for the December 30 final, continued their winning run with a 21-8 victory over the Dragons at Abercynon.
Tries from Harley Thomson and Matthew Roberts helped the home side into a 14-3 interval lead with Thomas Young adding a third for good measure in the second half.
Lewis Williams added three conversions whilst for the Dragons, there was a first half penalty from Sam Bevan and a second half try from Thomas Ashmead.
The 18s now take on the Ospreys at Merthyr on Monday whilst on Sunday, the Dragons entertain Gogledd Cymru at under 16 and 18 level. The 16s will play at Newbridge and the 18s at the Recreation Ground, Tredegar.