It was the Scarlets who opened the scoring with only quarter of an hour on the clock as flanker Will Boyde crossed the whitewash with outside half Dan Jones converting.
The Blues struck back soon after with a converted try bringing the scores level at seven points apiece.
Scarlets wing Josh Adams added the second try with Jones converting before the Blues struck back once again with a try for Tom Williams.
The visitor's went ahead for the first time at 54 minutes with a penalty for Jarod Evans and the Scarlets failed to regain the lead after that despite coming within a point after a try by flanker Josh Middleton.
Final score Scarlets 21 Blues 25
The Scarlets will travel to Llandarcy on Monday 17th September to take on the Ospreys.