Andy Fenby touched down twice as the Scarlets claimed a 26-14 bonus-point win over the Blues at Cardiff City Stadium on Saturday.
The only score of a frustrating opening period came though the visiting winger, but the game improved in the second half.
Fenby added his second before Alex Cuthbert reduced the deficit. However, further tries from Scarlets' scrum-half Gareth Davies and lock Dominic Day sealed the win, despite Casey Laulala's consolation touchdown.
The result means the Scarlets move to within one point of the play-offs and severely dents Cardiff's top-four aspirations.
The game started off with vibrancy, but descended into a dour, scrappy affair as both Welsh regions continually turned over ball.
While some internationals were given a rest, the likes of Scarlets' Scott Williams were allowed a chance to impress, but his match lasted just 30 minutes because of a shoulder injury.
But Williams' replacement, another impressive Welsh three-quarter, Jonathan Davies, helped set up the opening score. Four minutes from the half-time whistle Davies chipped the ball though and, after Cuthbert had made a hash of the clearance, Fenby went over.
Things improved in the second period and the Scarlets extended their advantage when a Dan Parks' mis-kick was pounced on by Fenby.
Aled Thomas converted both, but the Blues hit back when Cuthbert intercepted a pass and sprinted clear from 50 metres out with Parks adding the extras.
However, scrum-half Davies went over before Laulala crossed the whitewash as the Blues struck back almost immediately. Parks once again converted, but Day's try completed the victory and boosted the visitors' chances of qualifying for the top four.
For Cardiff Blues:
Tries: Cuthbert, Laulala
Cons: Sweeney 2
Tries: Fenby 2, G Davies, Day
Cons: Thomas 3
Yellow card: Lamont (Scarlets - 50th min)
Cardiff Blues: 15 Chris Czekaj, 14 Alex Cuthbert, 13 Casey Laulala, 12 Dafydd Hewitt, 11 Tom James, 10 Dan Parks, 9 Lloyd Williams, 8 Xavier Rush (c), 7 Josh Navidi, 6 Michael Paterson, 5 James Down, 4 Macualey Cook, 3 Scott Andrews, 2 Ryan Tyrell, 1 John Yapp.
Replacements: 16 Marc Breeze , 17 Nathan Trevett, 18 Sam Hobbs, 19 Cory Hill, 20 Maama Molitika, 21 Richie Rees, 22 Ceri Sweeney, 23 Gavin Henson.
Scarlets: 15 Liam Williams, 14 Sean Lamont, 13 Gareth Maule (capt), 12 Scott Williams, 11 Andy Fenby, 10 Stephen Jones, 9 Gareth Davies, 8 Ben Morgan, 7 Johnathan Edwards, 6 Josh Turnbull, 5 Damian Welch, 4 Dominic Day, 3 Deacon Manu, 2 Ken Owens, 1 Rhodri Jones.
Replacements: 16 Matthew Rees, 17 Phil John, 18 Peter Edwards, 19 Sione Timani, 20 Kieran Murphy, 21 Liam Davies, 22 Aled Thomas, 23 Jon Davies.
Referee: Nigel Owens (WRU)