The Blues, with their large and powerful pack, controlled the first half with the Scarlets' game blunted by numerous handling errors.
Using their forwards to their advantage, both Blues' tries came from powerful driving mauls which allowed their backs to score wide out.
Finding themselves 10-0 down, the Scarlets responded with a try from wing Steffan Price after No. 8 Steffan Taffestauffer secured possession, ran 60 metres and fed Price who scored wide out.
With a five point lead at the interval, the South were looking to pull further clear but a stop-start second period did neither side any favours.
The referee's whistle played a big part and at one point the South were reduced to 13 players but they hung on for the victory despite failing to score after half-time.
Scarlets' wing Kieran Jones kicked a penalty but then missed a penalty which would have given his side the lead and the match in the dying minutes as the South held on.
Final score Scarlets 8 Blues South 10