Cardiff Blues hung on to secure a well-deserved Guinness PRO14 double over the Scarlets after Matthew Morgan celebrated his 100th appearance with a fine try.
The capital city outfit led 10-0 at the break thanks to the full-back’s try, which was created by Jarrod Evans, who added a further five from the boot.
Evans and Halfpenny exchanged second-half penalties before Blade Thomson crossed to set up a nail-biting finale but the Blues held on.
It was just rewards for Cardiff Blues, who made an electric start to the encounter and displayed an abundance of grit and determination in an equally impressive defensive shift.
The victory sees the Blues climb above the Scarlets in the Conference B table but with the advantage of an extra game.
It had been 3,549 days since Dai Young had last taken charge of a Cardiff Blues side and he had to wait even longer after being caught in the aftermath of a serious accident on the M4.
The Cardiff Blues interim director of rugby had been on his way to Parc y Scarlets on Friday afternoon when traffic ground to a halt and become one of hundreds trapped on the motorway for hours.
Following a lively start to the encounter in bitterly cold conditions, it was the hosts that had the first opportunity to score. They turned down a shot at goal and went to the corner and following a slow, ambling drive they almost crept over.
The hosts were eventually ground to a halt but they were playing with advantage and went back to the corner. It appeared ominous but Ryan Elias overthrew, allowing Liam Belcher to break and Tomos Williams to clear.
After weathering the early storm, it was Cardiff Blues turn to show their attacking intent and once again they threatened the Scarlets from all areas with their willingness to run.
Willis Halaholo and Matthew Morgan made half breaks and clever kicking from Jarrod Evans pinned the Scarlets back before the opening try.
It was that man Evans, who caused the Scarlets so many problems two weeks ago, that unlocked the defence once more. Forget a double pump, this was more like a quadruple pump pass as he fixed Gareth Davies and Blade Thomson and released Morgan through the smallest of gaps.
The diminutive full-back, making his 100th appearance for the region, needed no second invitation as he seized the opportunity and scampered over with three defenders trailing in his wake.
Evans added the extras but the Blues were soon dealt a blow as their captain, Cory Hill, was forced off with a leg injury.
The Scarlets were enjoying plenty of ball in the opening quarter but they were met by a ferocious Cardiff Blues defence oozing intensity.
Cardiff continued to attack from all angles in the scrappy affair and threatened the Scarlets once more on 25 minutes. Tomos Williams took a quick tap to maintain momentum and Dai Young’s side went through the phases. A try did not appear too far away but Wyn Jones swooped for a turnover penalty to ease pressure.
As the half hour mark approached, Cardiff Blues laid siege to the Scarlets line with a powerful driving line out. They were held out and after playing with advantage, Halaholo flung a huge floating pass to the wing. It just about made it to Amos but he was unable to collect the ball from his laces.
The Blues went back to the corner and piled on more pressure before eventually settling for an Evans penalty.
Young would have been delighted when he arrived two see a 10 point lead on the scoreboard.
Despite further spells of pressure the score remained unchanged and Wales’ Capital Region entered the interval with a deserved 10-0 lead.
They made a bright start to the second half but a marginally high tackle from Evans, on Johnny Williams following a superb spot blitz, allowed the Scarlets a foot-hold.
Despite one turnover of possession the hosts remained camped in the Blues 22 for more than five minutes and threw the proverbial sink. They went through phase after phase of possession with their giant pack carrying half but they were unable to break the Cardiff spirit.
They eventually settled for a penalty, which former Cardiff Blues star Leigh Halfpenny slotted, to make it 10-3 after 50 minutes.
Shortly after there was an extended break in play as Scarlets talisman Sione Kalamafoni was knocked out. The number eight got his head on the wrong side of Dmitri Arhip and connected his the Moldovan’s hip and forearm.
The Blues attacked from the ensuing scrum but were twice tackled into touch in quick succession as the Scarlets survived.
Josh Navidi made his long-awaited return to action on 55 minutes, replacing Alun Lawrence, who had done a fine job at number eight in perhaps his biggest test yet.
A fine kick from Evans, and an equally impressive chase from Josh Adams, saw the Scarlets lose the ball in their own 22 and on 57 minutes another penalty was secured.
Evans bisected the posts to restore a 10-point advantage. As the hour mark passed, it appeared as though the Scarlets had claimed a crucial try.
With their heavy pack flexing their collective muscle, Cardiff Blues endured long spells of sustained pressure and playing with advantage, Dan Jones sent a scrappy cross kick to the right.
It landed in no man’s land with Morgan scrambling to collect but unable to secure the ball. The bounce evaded Adams and fell into the hands of Angus O’Brien for a simple run in.
The Scarlets marched back but replays showed Johnny McNicholl’s desperate dive had taken Morgan out and the try was chalked.
Glenn Delaney’s men turned the screw once more and Jones reverted to the same tactic, but without the luxury of advantage and both McNicholl and then Halfpenny were bundled to the ground before Botham pounced on possession.
The Scarlets had clearly spotted something as they went to the cross-kick on a third occasion and this time caught the Blues off guard. Blade Thomson was patrolling the touchline and he collected Stuff Hughes’ well-placed kick to cross in the corner.
Halfpenny converted from the touchline to set up a tense finale with just three points between the teams and just over ten minutes to play.
The would have been plenty of nervous viewers at home when Josh Turnbull, making his 200th appearance in the PRO14 to go fifth on the all-time list, was penalised at the breakdown and Dan Jones went the ball deep into the Cardiff Blues half. But, just went it mattered most, it was the experienced forward who rose to steal the lineout against the head.
The final moments saw Cardiff Blues camped deep in the Scarlets' 22, with replacement Ben Thomas coming close to breaking the hosts' defensive line with a half break and offload, but his attempted offload to Aled Summerhill was deliberately knocked on by Kieran Hardy, who was sent to the sin bin.