The Scarlets claimed their first victory at the Cardiff City Stadium and ended the Blues' unbeaten home record with a 16-10 success.
A try, conversion and penalty from fly-half Ceri Sweeney gave the hosts a 10-9 interval lead, Rhys Priestland kicking three penalties for the Scarlets.
But veteran Wales fly-half Stephen Jones came off the bench for the Scarlets to create a superb match-winning try for Priestland.
Both sides were coming off defeats on the road in the Heineken Cup and a shortage of confidence from both sides led to an error-strewn evening.
The place-kicking was hit and miss throughout and the pattern was set in the opening quarter, when the Blues Sweeney and Scarlets opposite number Priestland each kicked one and missed one, while Blues wing Leigh Halfpenny was wide with a long distance effort.
The visitors lost winger Morgan Stoddart to the sin-bin after a couple of dangerous tackles and the Blues took advantage when lively scrum-half Richie Rees darted through a half-gap and offloaded for Sweeney to touch down next to the posts, making his conversion a simple matter.
Referee Tim Hayes then evened the numbers up with a yellow card for retaliation by flanker Andries Pretorius, but Priestland was off target with the resultant penalty.
The Scarlets acting skipper, Sean Lamont, grabbed a loose ball deep in his own half, sparking a move which saw the newly-returned Stoddart held up just short.
The visitors kept up the pressure, but prop Iestyn Thomas was ruled by the television official to have lost the ball as he touched down.
After the Blues again lost a man - scrum-half Rees, carded for a deliberate knock-on - Priestland further punished them with another penalty.
The 14-man Blues survived early Scarlets pressure on the restart, before the game settled into an attritional battle as unforced handling errors too often ended attempts at creative back play.
But shortly after the hour mark, the visitors produced the game's one moment of flair.
Jones, brought into the action to provide some control, floated a kick towards the left touchline, the giant Lamont palmed it inside and Priestland, now at full-back, raced through to score. Jones converted.
A sharp break by centre Dafydd Hewitt gave the Blues some hope, but the move broke down.
Priestland was wide with a penalty from inside his own half, but, despite losing replacement lock Dominic Day to a last-minute yellow card, the Scarlets held on to go second in the Magners League table.
Cons: S Jones
Pens: Priestland 3
Cardiff Blues: 15 Chris Czekaj, 14 Leigh Halfpenny, 13 Casey Laulala, 12 Dafydd Hewitt, 11 Tom James, 10 Ceri Sweeney, 9 Richie Rees, 8 Xavier Rush (c), 7 Sam Warburton, 6 Andries Pretorius, 5 Michael Paterson, 4 Bradley Davies, 3 Scott Andrews, 2 T. Rhys Thomas, 1 Gethin Jenkins.
Replacements: 16 Rhys Williams, 17 John Yapp, 18 Taufa'ao Filise, 19 Paul Tito, 20 Maama Molitika, 21 Lloyd Williams, 22 Tom Shanklin, 23 Richard Mustoe.
Scarlets: 15 Daniel Evans, 14 Morgan Stoddart, 13 Jon Davies, 12 Gareth Maule, 11 Sean Lamont (c), 10 Rhys Priestland, 9 Tavis Knoyle, 8 Rob McCusker, 7 Johnathan Edwards, 6 Josh Turnbull, 5 Damian Welch, 4 Lou Reed, 3 Deacon Manu, 2 Ken Owens, 1 Iestyn Thomas.
Replacements: 16 Matthew Rees, 17 Rhodri Jones, 18 Peter Edwards, 19 Vernon Cooper, 20 Ben Morgan, 21 Martin Roberts, 22 Stephen Jones, 23 Regan King.
Referee: Tim Hayes (Wales)