The Blues trailed 18-3 following a frustratingly one-sided first-half in which nothing went right.
But they refused to give in and gave themselves a chance of completing a stunning comeback as Halaholo and Williams crossed within five minutes of each other.
However, the Scarlets quickly responded through Jonathan Evans before namesake Steffan compounded Cardiff Blues’ woes.
It took the hosts less than a minute to open the scoring at Parc y Scarlets as the Blues were harshly penalised and former favourite Leigh Halfpenny slotted the simple penalty.
Wales’ Capital Region quickly had the opportunity to respond but they turned down the shot at goal and went to the corner. They launched a driving lineout but when the Scarlets infringed once more they settled for the points
The frenetic opening to the encounter continued as the ball came free from the restart allowing the Scarlets to go on the attack.
Cardiff Blues just about clung on as the ball was moved left but they ran out of numbers as the Scarlets spread the ball back to the right with Parkes ghosting over.
Halfpenny pushed the conversion to the left but the try had given his team an 8-3 lead after just seven minutes.
The full-back made amends on 14 minutes after the Blues were again harshly penalised by Stuart Berry at the breakdown.
Cardiff Blues displayed plenty of intent but simply did not get the rub of the green on several occasions as refereeing decisions or 50-50 balls failed to go their way.
They were dealt another blow when Nick Williams was sent to the sin-bin for over-zealous contact at the breakdown and to further compound their woes, Jarrod Evans was soon forced off for treatment.
There were still some opportunities for the visitors with Rey Lee-Lo causing the hosts problems but following one midfield bust, Tomos Williams was unable to scoop the ball from his shoelaces.
The Scarlets however, remained in control and they threatened once again down the left. Steffan Evans won the foot race to a kick through and Scott Williams collected the ball to dive over the whitewash.
Following minutes of television replays, Stuart Berry finally ruled the try out but bizarrely penalised Alex Cuthbert.
And the Scarlets needed no second invitation as they kicked to the corner and rumbled over with Ryan Elias collecting the try. Halfpenny added the extras to make it 18-3 with half-time approaching.
Halaholo was unfortunate not to force his way over in the closing stages of the half but as he wriggled for the line with defenders piling up, the ball was lost and the score remained unchanged at the break.
The Scarlets continued to frustrate the Blues in the second-half with the game still failing to ignite.
But the opening eventually came for Wison’s men on 53 minutes with Shingler making a half break and finding Halaholo, who waltzed home.
Shingler, who remained on for Evans, added the conversion to bring the scores to 18-10 and he was back at the tee five minutes later as Cardiff Blues claimed quick-fire tries.
The opportunity came after Shingler hacked through the ball and Johnny McNicholl failed to clear. The Kiwi wing ran his way into trouble and Williams stripped the ball to wriggle over.
Shingler converted from wide to bring Cardiff Blues within a point but the hard work was quickly undone as the home side pounced.
Parkes moved the ball superbly out of contact to Evans, who kicked through and despite plenty of cover, the ball bounced up favourably for Jonathan Evans to crash over.
Rhys Patchell failed to slot the conversion but it had little impact as the Llanelli-based region displayed their ruthless edge with a fourth try, this time Steffan Evans the scorer.