Danny Wilson’s men made a nightmare start in front of 9,442 spectators as Aaron Shingler scored after just seven minutes.
Tom James snatched an interception try but the Blues trailed 21-7 at the break.
Rhys Patchell created the bonus-point try after just 37 seconds of the second-half to leave the hosts starring down the barrel.
And despite fighting back with a spectacular Blaine Scully try, the hosts had left themselves with too much to do and were unable to convert opportunities under immense pressure.
The Scarlets enjoyed all the early territory and possession at a packed BT Sport Cardiff Arms Park.
They dominated the opening five minutes and despite an initial strong defensive showing, the pressure eventually told as Shingler stretched over.
Patchell added the extras upon his first return to his former stomping ground and he was soon back at the kicking tee as the Blues’ nightmare start continued.
A superb offload from DTH van der Merwe was collected by Jonathan Evans, who tore through the Blues for a quick-fire try, which Patchell converted.
Spurred on by the vociferous crowd, Cardiff Blues fought back. Cory Allen knocked on taking a flat pass but from the next play James picked off John Barclay’s pass to score his sixth try of the season.
Steve Shingler slotted the conversion from wide and Wales’ Capital Region continued to threaten in a frenetic period.
Rey Lee-Lo soon claimed another interception and released James, but Van der Merwe snaffled Macauley Cook’s pass. It looked as though he would race clear but Lee-Lo matched him for pace and with the help of Morgan saved the day.
However, there was no let off from the next interception in a helter-skelter encounter. With the Blues attacking the Scarlets’ try-line, Aled Thomas claimed Shingler’s pass and put Van der Merwe away.
Patchell knocked over the conversion to give his side a 21-7 lead on 25 minutes. Despite the set-back the Blues responded well and dominated for long periods.
Tomos Williams was held up over the line and from the ensuing scrum they continued to hammer the Scarlets line and Cam Dolan was held agonisingly short.
From the next set piece, the Blues won three consecutive penalties but at the fourth collapse they saw Taufa’ao Filise sent to the sin-bin with Wyn Jones, who had been pinged on the three previous occasions.
They continued to dominate the scrum and won two more penalties but a penalty try was not forthcoming.
It appeared as though Matthew Morgan had jinked over after the Blues went wide but he was held up and Lloyd Linton called for half-time to the dismay of the home crowd.
And the Blues were punished within 36 seconds of the restart by the clinical Scarlets as former crowd favourite tore through the home defence and put Hadleigh Parkes away.
Patchell was unable to add the extras but the try had given the visitors a healthy 26-7 lead. Shingler soon responded with a well-struck penalty but the Blues had a mountain to climb
Minutes later Shingler combined with James and he after feeding Cook, Kirby Myhill was released on the left. He bulldozed into the covering defence and the Blues were awarded a penalty.
Twice they dinked the ball into the corner but at the second lineout Myhill overthrew and Van der Merwe broke clear.
Finally, the Scarlets indiscretions were penalised with a yellow card, Will Boyd, the man sent to the sin-bin.
And after several minutes of attacking, the Blues finally found a breakthrough as Scully collected a long Jarrod Evans pass to dive acrobatically into the corner.
Shinger was unable to convert but minutes later he sliced through the Scarlets defence from a nEvans pass.
He beat two defenders but was eventually hauled down and the Scarlets earned a fortuitous penalty at the breakdown.
The Blues battled in vain for the remainder of the encounter, but were unable to secure a losing bonus point.