The Scarlets led 15-3 early in the second-half following tries from Steff Evans and Scott Williams.
Danny Wilson’s side controlled large periods of the encounter but struck on the counter-attack with Halaholo finishing with aplomb.
The encounter got underway at an electric pace with both teams demonstrating their willingness to keep the ball in hand and attack.
Both teams had opportunities in attack and following almost ten minutes of fast-paced action the Scarlets claimed the opening try thanks to Steff Evans.
It was former BT Sport Cardiff Arms Park favourite, Rhys Patchell that laid the foundations from the score with a blistering counter-attack. He was hauled down but the ball was recycled and with an overlap on the left Evans dummied his way over in the corner.
Patchell was unable to add the conversion and the Blues responded with venom as the thrilling encounter continued.
Matthew Morgan, Rey Lee-lo and Willis Halaholo were all prominent in attack but the Scarlets scrambled superbly in defence to snuff out any half chance.
Patchell added a penalty on 30 minutes, after the Blues were harshly penalised at the breakdown, but Jonathan Davies knocked on from the restart and Shingler quickly hit back.
The Scarlets almost pounced for an opportunist try inside the first minute of the second-half after Will Boyde hacked through and Scott Williams caught Alex Cuthbert off guard.
Rhun Williams made the try-saving tackle and despite having a vast overlap the Scarlets were unable to convert the opportunity as the Blues scrambled for their lives.
The hosts were eventually rewarded with a penalty but Patchell screwed the simple penalty wide.
However, they soon claimed a second try through Jonathan Davies after some fine work from Scott Williams and Evans, again on the counter-attack. Patchell converted the effort to extend his side’s lead to 15-3.
A rampaging charge from Kristian Dacey on 50 minutes put the Scarlets on the back-foot and it appeared as though Rhun Williams could score. He beat his opposite number but could not go around Aled Thomas and lost the ball forward.
But the Blues did claim a try five minutes later after scrum-half Williams and Shingler combined on the left. Shingler released Williams down the wing and he then passed inside to Halaholo.
The centre still had plenty to do but he displayed a devastating step to leave Patchell grasping at air before scoring in the corner.
Shingler converted from the touchline and the Blues almost claimed a quick-fire second after Josh Navidi burst through the defence. He had Cuthbert and Tomos Williams in support either side but Johnny McNicholl’s gamble paid off and he intercepted.
The New Zealand winger very nearly broke clear but the Blues recovered to quell the attack.
Wilson’s side continued to battle in the closing stages but were unable to manufacture a breakthrough.
The Scarlets then had a late opportunity to deny the Blues a bonus-point but Thomas was twice off target, first with a penalty and then a snatch drop goal.