|Pro14: Scarlets v Cardiff Blues|
|Scarlets (0) 10|
|Tries: Thomson Cons: Halfpenny Pen: Halfpenny|
|Cardiff Blues (10) 13|
|Tries: M Morgan Cons: J Evans Pens: J Evans 2|
Cardiff Blues completed the Pro14 double over Scarlets with victory in Dai Young's first game back in charge.
Blues were never behind following a Matthew Morgan try and eight points from fly-half Jarrod Evans.
Scarlets responded with a Blade Thomson try and five points from Leigh Halfpenny's boot.
But Wales coach Wayne Pivac will be concerned as Six Nations squad members Cory Hill, Wyn Jones and Tomos Williams were forced off through injury.
In contrast, Jake Ball, Ken Owens and Josh Navidi all returned to action after being named in the Six Nations squad.
Interim director of rugby Young missed the start of the first game of his second spell in charge after being stuck in traffic for more than four hours, after the M4 motorway was closed because of a serious accident.
Young has taken over from departed head coach John Mulvihill with this derby, brought forward from early March following the cancellation of the European group games.
Scarlets were looking to avenge a 29-20 defeat by the same opposition two weeks ago, and had made six changes.
Wing Johnny McNicholl replaced the suspended Liam Williams who had been sent off in the defeat. McNicholl was joined in the starting side by returning internationals Wyn Jones, Ryan Elias and Ball.
Scarlets flanker Josh Macleod was the only uncapped player named by Pivac, but was ruled out of this game with a minor ankle problem. Ed Kennedy lined up at openside with Scarlets also missing injured trio James Davies, Jac Morgan and Dan Davis.
Macleod was due to be directly up against the impressive James Botham, who was omitted after winning three Wales caps in the 2020 autumn campaign. Botham proved his point to the watching Wales management with a monumental man-of-the-match performance.
It was a magical moment from another returning Welsh squad member Jarrod Evans with the fly-half producing a deft offload to set up a try for full-back Morgan on his 100th Cardiff Blues appearance.
Cardiff Blues suffered a blow when they lost their captain and Wales lock Hill to an upper leg injury.
Scarlets again proved stuttering in attack and their lineout struggles continued in a lacklustre first-half performance.
Young arrived in time to watch Evans increase the Blues lead with a simple penalty to 10 points at half-time.
The visitors defensively dominated their opponents with Samoa centre Rey Lee-Lo proving the most influential attacker on the field.
Scarlets started the second-half more encouragingly and tested the visitors defence before slotting over a Leigh Halfpenny penalty.
The hosts welcomed the return of captain Owens after three months on the sidelines following shoulder surgery.
Scarlets suffered a double blow when prop Wyn Jones suffered a neck problem and prominent Tonga number eight Sione Kalamafoni was forced off a head injury.
Jones was only just returning after recovering concussion, while Kalamafoni's injury was sustained when trying to tackle Cardiff Blues prop Dmitri Arhip whose elbow caught the Scarlets number eight in the face, an incident the officials judged as legal.
Navidi returned to action for the first game of the season since four months out with concussion as he entered the field with fellow Wales forwards Rhys Carre and Dillon Lewis.
There was more concern for Pivac when Blues scrum-half Tomos Williams was forced off with an arm injury before Evans slotted over a penalty to restore the 10-point advantage.
Scarlets were denied a try for replacement Angus O'Brien after McNicholl pushed Morgan when chasing a cross kick.
That tactic eventually paid dividends when Scotland flanker Thomson latched onto a kick from replacement Steff Hughes for a try Halfpenny converted majestically from the touchline to reduce the deficit to three points.
Replacement Scarlets scrum-half Kieran Hardy was yellow-carded in the final minute for a deliberate knock-on as the visitors ground out the victory.
Scarlets head coach Glenn Delaney: "Frustration is one emotion you can talk about. When we have four or five chances on the goal-line the expectations are we are going to turn those into points and we didn't tonight.
"We have to be better in those moments and we had enough territory and possession to score more points.
"You are always worried if you lose two in a row but I would be far more aggravated if we were not creating chances to score. We just have to be better in the final five or six metres and we have to take our chances."
Cardiff Blues interim director of rugby Dai Young: "You can never underestimate a win down here because it's a tough place to come. They have quality players who are well-coached so you have to savour every win down here.
"Defensively we were in good shape all night but I felt we gave away too many penalties in the second-half. I was pleased with the attitude and commitment though and it was a win built on character and defence."
Scarlets: Leigh Halfpenny; Johnny McNicholl, Jonathan Davies (capt), Johnny Williams, Steff Evans; Dan Jones, Gareth Davies; Wyn Jones, Ryan Elias, Javan Sebastian, Jake Ball, Sam Lousi, Blade Thomson, Ed Kennedy, Sione Kalamafoni.
Replacements: Ken Owens, Phil Price, Werner Kruger, Tevita Ratuva, Uzair Cassiem, Kieran Hardy, Angus O'Brien, Steff Hughes.
Cardiff Blues: Matthew Morgan; Josh Adams, Rey Lee-Lo, Willis Halaholo, Hallam Amos; Jarrod Evans, Tomos Williams; Corey Domachowski, Liam Belcher, Dmitri Arhip, Seb Davies, Cory Hill (capt), Josh Turnbull, James Botham, Alun Lawrence.
Replacements: Kristian Dacey, Rhys Carré, Dillon Lewis, Rory Thornton, Josh Navidi, Lloyd Williams, Ben Thomas, Aled Summerhill.
Referee: Adam Jones (WRU)
Assistant referees: Dan Jones, Ben Whitehouse (both WRU)
TMO: Ian Davies (WRU).