Left-winger Andy Fenby was the beneficiary each time and even when the Blues cut the deficit to five points midway through the second half the visitors always looked capable of sealing their victory.
With Wales still awash with celebration after last weekend's Grand Slam and the international heroes returning to their regions, a large crowd might have been expected on a warm spring evening. But only four men involved in last Saturday's victory over France started, three of them replacements at the Millennium Stadium.
And a meagre crowd of 8,337 were present to see an error-strewn opening before Aled Thomas, a late inclusion at fly-half when Stephen Jones pulled out with illness, missed a straightforward penalty for the Scarlets.
The home side were handed a chance to break the deadlock after half an hour, but Dan Parks was similarly wayward with his penalty attempt. Jonathan Davies replaced Scott Williams in the Scarlets midfield and promptly placed a kick behind the Blues defence.
Blues wing Alex Cuthbert beat Gareth Maule to the ball, but fumbled a yard from the try-line, allowing the supporting Fenby to dot down for an easy score. It was converted by Thomas. Cuthbert made some amends when he beat Fenby to another hack downfield, this time successfully claiming the loose ball to keep the half-time margin to seven points.
But the Blues shot themselves in the foot again early in the second half, when Parks completely miskicked a rolling ball and Fenby once more had the simplest of jobs to pick up and dive over. Thomas converted superbly from the touchline.
The Cardiff side were given some hope when Sean Lamont was sin-binned for a deliberate knock-on and the Scarlets were even more generous when a careless Thomas pass was snatched from the air by Cuthbert, who raced 50 yards to the posts. Replacement fly-half Ceri Sweeney added the extras.
But the visitors were not to be denied, capitalising when a Blues move broke down, Lamont sending scrum-half Gareth Davies jinking past two defenders to touch down.
In a thrilling last quarter Blues centre Casey Laulala weaved his way over near the posts, Sweeney converting, but George North came off the bench to break two tackles and put lock Dominic Day over for the bonus-point try.
Connacht had the better of possession and territory but Munster still managed to win a rip-roaring RaboDirect PRO12 derby 20-16.
Chasing their first interprovincial win since April 2010, Connacht foraged into a 10-6 half-time lead thanks to a converted try from hooker Ethienne Reynecke.
The 7,022 attendance, a record crowd for a league game in Galway, watched Simon Zebo zip though for a levelling try with just over 20 minutes left.
Miah Nikora, who finished with 11 points, kicked Connacht back in front only for flanker Tommy O'Donnell's 67th-minute try to seal it for the visitors.
Adam D'Arcy scored a dramatic injury-time try as Ulster claimed a 27-23 win against Benetton Treviso in the RaboDirect PRO12 to sit just outside the top four.
Scrum-half Ruan Pienaar kicked the conversion, taking his tally to 12 points, as Ulster came back to snatch a thrilling victory.
Ulster had trailed going into the last 10 minutes, but first Paddy Wallace claimed a try and then D'Arcy crossed, with Pienaar converting both.
Treviso's outside-half Kris Burton was also on fine form with the boot as he kicked a penalty, two conversions and two drop-goals, but it proved ultimately not enough.
The home side went ahead after two minutes as hooker Enrico Ceccato touched down and Burton converted to lead 7-0, but the visitors struck back when Pienaar kicked a penalty after 17 minutes.
Five minutes later the visitors went ahead through a Chris Henry try which was converted by Pienaar for Ulster to lead 10-7.
Burton kicked a drop goal and Pienaar a penalty before the home side took control.
Burton slotted over another penalty and a second drop goal before converting a try from Alberto Di Bernardo with 12 minutes remaining.
But the visitors were not finished and both Wallace and D'Arcy scored tries in the closing minutes, with D'Arcy's coming in injury time.
With both efforts converted by Pienaar the visitors claimed victory.