Scrum-half Martin Roberts scores the Scarlets' first try against the Blues
The Scarlets registered their first win at new home Parc y Scarlets as they upset the form-book to beat the Blues in the Magners League.
On Friday the Dragons beat the Ospreys and the Blues also suffered as they rested a glut of front-line stars.
Scrum-half Martin Roberts and prop Phil John scored home tries with Stephen Jones' boot doing the rest.
Blues wing Richard Mustoe crossed. Ceri Sweeney and Leigh Halfpenny kicked eight points between them.
Halfpenny started at full-back in a weakened Blues side while Wales internationals Martyn Williams, Gethin Jenkins, Tom Shanklin and Andy Powell were rested along with Kiwi pair Ben Blair and Paul Tito.
Young scrum-half Darren Allinson started his third game, while there was a new second-row partnership Scott Morgan and Bradley Davies as the Blues attempted to continue the form that saw them beat Heineken Cup rivals Biarritz twice on the bounce.
Kiwi lock Simon Maling passed a fitness test to take his place in the Scarlets pack as they sought to bounce back from the disappointment of back-to-back Heineken Cup defeats to Ulster.
Halfpenny was one of the outstanding first-half performers, his deft touches and sure approach helping him stand out in attack and defence.
Scarlets' fly-half Stephen Jones opened the scoring after nine minutes, punishing flanker Robin Sowden-Taylor going offside at a ruck.
Halfpenny replied two minutes later and after fly-half Jones and opposite number Ceri Sweeney missed three shots at goal between them, the home number 10 put his side only for his rival to level a minute later.
After John Yapp's off-the-floor pass was intercepted by Regan King a home try seemed inevitable, but Halfpenny snuffed out the threat with a superb tackle.
However, there was no stopping the Scarlets from making amends after home full-back Rhys Priestland launched a spectacular long-range counter-attack that ended with scrum-half Martin Roberts diving over.
Blues centre Jamie Robinson was sin-binned shortly after the break and if Roberts' try was the game's highlight from a backs' perspective, home prop Phil John's close-range effort was one for the admirers of pack-play.
The visitors struck back with a Richard Mustoe try after 62 minutes, the wing appearing to the Scarlets and their fans to have taken a forward pass.
However, Ceiron Thomas was also at fault as his weak tackle allowed Mustoe to burst clear.
Sweeney's conversion put the Blues back within five points, but Stephen Jones' three late penalties kept the visitors at bay.
Scarlets boss Nigel Davies:
"I was very, very pleased with the way our players went about their business. I thought it was a very professional performance under obviously difficult circumstances.
"There was a lot of pressure on this team to perform and get a result tonight [Saturday] and we did it, you know.
"We played in the right areas, which is something we haven't in the past and we squeezed that victory and that's what we needed to do to build confidence and momentum in this team."
Blues head coach Dai Young:
"To be honest with you in the second-half we weren't really in it, were we?
"I think the Scarlets in fairness - you've got to give them a lot of credit - I thought they totally controlled field position.
"We never had any possession in an area where we could mount any attack so basically for most of that game we defended, which was disappointing, really.
"We never really got into our stride… but you've got to give the Scarlets a lot of credit. I thought they controlled the game."
Scarlets: Rhys Priestland; Ceiron Thomas, Regan King, Jonathan Davies, Mark Jones; Stephen Jones, Martin Roberts; Phil John, Matthew Rees, Deacon Manu, Vernon Cooper, Simon Maling, Simon Easterby (c), Dafydd Jones, David Lyons. Replacements: Lee Williams for Davies (61), Ken Owens for Rees (69), Craig Cross for Manu (80), Dominic Day for Maling (61), Johnathan Edwards for Easterby (68).
Not Used: Lou Reed, Gavin Cattle.
Blues: Leigh Halfpenny; Richard Mustoe, Jamie Robinson, Gareth Thomas, Tom James; Ceri Sweeney, Darren Allinson; John Yapp, Rhys Thomas, Taufa'au Filise, Bradley Davies, Scott Morgan, Ben White, Robin Sowden Taylor (c), Maama Molitika. Replacements: N. Robinson for Halfpenny (52), D. Jones for White (63), Czekaj for James (72).
Not Used: Gary Powell, Gareth Williams, Damian Welch, Jason Spice.
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