The Scarlets put a severe dent in the Blues' title ambitions
Scarlets: (14) 38
Tries: Watkins (2), Davies (2)
Cons: S Jones (3)
Pens: S Jones (4)
Blues: (0) 10
The Blues' big-game away woes struck again as defeat at the Scarlets gave control of the league to Leinster.
Con: N Robinson
Pen: N Robinson
An awful first half from the visitors saw them 14-0 down, punished by a Matt Watkins try and Stephen Jones' boot.
Substitute Nicky Robinson inspired a second-half fightback, a Xavier Rush try helping reduce the gap to seven.
But as the Blues increased the pressure Rhys Williams threw Barry Davies an interception try, Davies and Watkins completing braces in the dying minutes.
Leinster visit Cardiff Arms Park for a crunch showdown with the Blues on Friday, but the Scarlets' win also increases the title chances of the Ospreys, who travel to the Dragons and the Border Reivers next week.
Stephen Jones was outstanding and the balance between our kicking and moving game was spot on
Scarlets director of rugby Phil Davies
The Scarlets' win guarantees Heineken Cup rugby for them next season, as the Dragons are now certain to be the bottom Welsh region.
The Scarlets, thriving in the fine, sunny conditions, came out of the blocks as if it was them going for the league, Jones striking an early penalty.
The fired-up home pack drove the Blues - who have never won at Stradey Park - off their own scrum ball in centre field, giving the Scarlets the chance of a swift counter attack.
Regan King was quick to seize the opportunity, striding easily outside Jamie Robinson and feeding wing Watkins, who cantered in for a fine score.
The Llanelli side maintained the pressure, Jones adding two more penalties and seeing Blues flanker Martyn Williams sin-binned for killing the ball.
The visitors stepped up the pace after a half-time ear-bending, and saw Scarlets prop Iestyn Thomas sin-binned for an offence at the breakdown.
Outside-half Nicky Robinson - a second-half replacement for Ben Blair, with Nick Macleod moving to full-back - slotted the resultant penalty to get his side on the scoreboard.
But from the restart a seven-man Scarlets scrum won a penalty, Jones extending the lead back out to 14 points.
Chris Czekaj made a superb midfield break from Tom Shanklin's inside pass, but the Wales wing spurned a try-scoring pass to Jamie Robinson.
Instead, he floated a more difficult long ball to Williams, who let the ball slip from his grasp.
The Cardiff side's second-half dominance continued, though, and after a series of short-range mauls were collapsed Scott MacLeod was sent to the sin bin.
We are still in with a shout and would have settled for this before Christmas
Thomas returned soon afterwards, but Blues captain Rush picked up from the base of a five-metre scrum to stretch for the line, Nicky Robinson's conversion bringing his side within a score.
Davies and Jones each missed penalty shots to give the home side more breathing space, and the visitors looked set to punish them with a sustained attack.
But with a four-to-two overlap in the shadow of the Scarlets' posts, replacement Davies rushed up on Williams' looped pass for an 80m interception try.
Good line-out ball and excellent work from Gavin Evans then laid the platform for Davies to score in the corner.
And the disheartened Blues were ripped apart by another superb break from Evans, King supporting before sending Watkins haring for the line.
Scarlets director of rugby Phil Davies:
"That was a great performance, very pleasing - we started very well and proved very resilient in the second half.
"We were positive throughout and our movement was good.
"Stephen Jones was outstanding and the balance between our kicking and moving game was spot on."
Blues coach Dai Young:
"We came into the game with momentum and our destiny in our own hands, but looked tired and one-paced.
"All the games had caught up with us and the interception try was the nail in the coffin.
"We can do ourselves a favour against Leinster, but the Ospreys could still win it - we have to hope the Gwent mafia [Dragons] can do us a favour on Tuesday.
"We are still in with a shout and would have settled for this before Christmas."
Scarlets: Stoddart, Daniel, King, G. Evans, Watkins, S. Jones, Peel, I. Thomas, Rees, Manu, Cooper, MacLeod, Easterby, G. Thomas, Popham.
Replacements: B. Davies for Stoddart (39), C. Thomas for Daniel (41), A. Jones for Cooper (41), D. Jones for Popham (65). Not Used: J. Davies, Owen, Stuart-Smith.
Blues: Blair, R. Williams, J. Robinson, Shanklin, Czekaj, Macleod, Phillips, Jenkins, Thomas, G. Powell, Davies, Jones, Morgan, M. Williams, Rush.
Replacements: N. Robinson for Blair (41), Stcherbina for Shanklin (56), Roberts for G. Powell (79), Lewis for Morgan (70), Sowden-Taylor for M. Williams (78). Not Used: G. Williams, Evans.
Referee: Tim Hayes (WRU).