|Scarlets (9) 22|
|Tries: Barclay Cons: Thomas Pens: Jones (4), Thomas|
|Cardiff Blues (18) 28|
|Tries: Williams, James, Anscombe Cons: Anscombe (2) Pens: Anscombe (3)|
Cardiff Blues survived a late rally to complete a first double over Scarlets in six years and dent their hosts' Pro12 play-off hopes.
Fly-half Gareth Anscombe scored a try in an 18-point tally with Lloyd Williams and Tom James also crossing.
Blues dominated for an hour, but John Barclay's try and a yellow card for Josh Turnbull led to a frantic finish.
Dan Jones kicked 12 points for the Scarlets, who were a shadow of the team that beat Ospreys a week earlier.
Danny Wilson's Blues team improved their own chances of claiming a place in next season's European Champions Cup with an impressive display of counter-attacking rugby.
Scarlets were unable to establish the forward dominance which had undone the Ospreys.
Wilson's decision to go with three recognised openside flankers in his back row paid dividends as the Blues won six turnovers in the first half.
Anscombe and full-back Rhys Patchell made the most of the possession, wrong-footing the home defence as they quickly changed the point of attack.
Only Blues indiscipline kept the Scarlets in touch at half-time as Jones punished transgressions.
A change of tactics early in the second half saw Scarlets slow the pace and establish a little more control, but all seemed lost when man-of-the-match Sam Warburton halted a Scarlets counter, and James and Anscombe combined for a thrilling touchdown.
Scarlets finally crossed the whitewash when Barclay drove over from short range after Blues lost a defensive scrum.
Thomas' conversion got the Scarlets to within a converted try and Turnbull's yellow card gave them added impetus.
But Blues held-on for a deserved win.
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Scarlets: Michael Collins; Steff Evans, Regan King, Hadleigh Parkes, DTH van der Merwe; Dan Jones, Gareth Davies; Rob Evans, Ken Owens (capt), Samson Lee, Jake Ball, David Bulbring, John Barclay, James Davies, Morgan Allen
Replacements; Kirby Myhill, Dylan Evans, Peter Edwards, George Earle, Maselino Paulino, Aled Davies, Aled Thomas, Gareth Owen
Cardiff Blues: Rhys Patchell; Dan Fish, Garyn Smith, Rey Lee-Lo, Tom James; Gareth Anscombe, Lloyd Williams; Gethin Jenkins (c), Matthew Rees, Taufa'ao Filise, Josh Turnbull, James Down, Sam Warburton, Ellis Jenkins, Josh Navidi
Replacements: Kristian Dacey, Brad Thyer, Dillon Lewis, Jarrad Hoeata, Manoa Vosawai, Lewis Jones, Jarrod Evans, Aled Summerhill
Referee: Marius Mitrea (Italy)
Assistant referees: Wayne Davies, Greg Morgan (WRU)
Citing commissioner: Aurwel Morgan (WRU)
TMO: Jon Mason (WRU)