Following a dramatic last-gasp victory over the Dragons, the Blues travelled to Parc y Scarlets targeting their first victory in Llanelli since 2016.
They made a flying start to the tense encounter and led by 10-points thanks to a Lloyd Williams try and the 100 per cent kicking of Gareth Anscombe.
The Scarlets hit-back through Johnny McNicholl and dominated for long periods in either half but a colossal defensive effort from Wales’ Capital Region was decisive.
With both teams searching for the winning moment, Willis Halaholo stepped up shortly after the hour mark as he sliced through the home defence and put Rey Lee-Lo away.
Anscombe kept the scoreboard ticking as the Blues eased out of reach, and with Scarlets forced to play from deep, interceptions from Lee-Lo and the Welsh international outside half saw the visitors clinch the five points in the final moments, breaking the Scarlets' 26-match unbeaten home run in the Championship.
Cardiff Blues had the first opportunity to put points on the board as the Scarlets came in at the side at a driving lineout.
It was on the limits of Gareth Anscombe’s range but he bisected the posts from wide to give his side the lead and it did not take long to stretch it further.
A superb clearance from the restart by Lloyd Williams pinned the hosts back into their own half and when they spilt the ball at the lineout, the Blues were handed a golden opportunity to counter.
Possession was shifted right and Rey Lee-Lo sliced through the Scarlets defence before finding Williams on the inside.
The scrum-half put Blaine Scully on the outside and while the USA international was caught by a superb covering tackle by Jonathan Davies, he managed to offload to Williams before he slid into touch for a stunning team effort.
Anscombe slotted the difficult touchline conversion to make it 10-0 after little more than 10 minutes.
The try had put Mulvihill’s men in a commanding position but shortly after the restart, the Scarlets struck against the run of play.
Garyn Smith contested for the high ball and managed to flick it back but a nightmare bounce gifted possession to the Scarlets and a sublime offload put Johnny McNicholl away.
Prydie was unable to add the extras but the Scarlets were right back in the contest having weathered a Cardiff Blues storm.
It was now their turn to take the ascendancy and they enjoyed a sustained period of pressure, either side of the half-hour mark, as they laid siege to the Cardiff Blues try line.
Thankfully, Wales’ Capital Region responded with a series of gargantuan defensive sets as the Scarlets failed to breach the line.
The Llanelli-based outfit continued to dominate in the closing stages of the half but a pair of crucial turnovers from Josh Navidi elevated the pressure.
The tension continued following the break with both teams opting to kick the ball for territory rather than attack.
Indiscipline eventually gave the Scarlets the opportunity to attack but once again, in the face of enormous pressure, Cardiff Blues held strong.
With the decisions continuing to go the way of the home side, they once again went to the corner but the ball was lost. They continued to control territory until Olly Robinson pounced for a trademark turnover less than a minute after coming on.
With the game balanced on a knife-edge, it was likely one big moment would decide the encounter and Halaholo duly obliged on 62 minutes. The electric centre, beat Dan Jones on the outside with a typical hitch kick before drawing McNicholl to release Lee-Lo.
The Samoan international was caught by Jonathan Davies but managed to skip out of the tackle to send the Cardiff Blues travelling contingent into raptures.
Anscombe maintained his 100 per cent strike rate to open up a commanding 12-point lead with less than 20 minutes remaining.
However, with the hosts forced to play from deep, looking for a way back into the game, they were twice punished as Lee-Lo and Anscombe perfectly-read Scarlets' long passes to cross the whitewash and seal a late bonus point in Llanelli.