With scrum half Laura Prosser adding the extras, the Blues had got their noses in front and stayed there for the rest of the game, despite the Scarlets best efforts.
The Scarlets running game was kept in check by the Blues magnificent drift defence where winger Rachel Green stood out with several timely interventions.
No 8 Vikki Gaylard scored in the 62nd minute while hooker Lowri Harries sealed the deal three minutes from time with a touchdown.
Blues Team Manager Leila Hughes was delighted with the victory, saying: "The whole team was fired up for the game. It was a really tough, physical game but I'm delighted all the hard work the girls have been putting in at training paid off against the Scarlets."
Hughes's counterpart at the Scarlets, Sara Davies, said: "The Blues defence prevented us from playing and stopped our ball carriers. We now have to regroup for our next game against the Ospreys in three weeks time."
In the other game at Cross Keys, the Dragons were comfortable 25-3 victors over the Ospreys.
The game was tied up at 3-3 after Bethan Preece and Yasmin Leung had exchanged penalties but once Dragons skipper Jamie Kift touched down in the 11th minute, there was no way back for the young Ospreys.
Leila Johns and replacement Naomi Whybrow also scored tries to cement the victory with Leung compiling a total of 10 points with her boot to condemn the Ospreys to their second defeat in the championship.