The visitors had lost just one of their previous 11 fixtures against their fellow Welsh regions and they made sure they extended that run to 12, albeit in a largely forgettable game.
Neither team offered a significant attacking threat, and with aerial ping pong the order of the day, the game was dominated by the boot. Thomas kicked all three of his penalties in the first half, Rhys Patchell responding with an effort of his own.
And although the visitors failed to score after half time, they held on to the win despite another penalty from Patchell.
With both sides fielding a host of Welsh internationals, a sizeable Arms Park crowd expected an entertaining encounter. But in a scrappy opening 10 minutes, it was the visitors who dominated.
Thomas, who is set for a run of games with Wales playmaker Rhys Priestland out injured, gave the Scarlets a 6-0 lead with two early penalties.
Thomas’ opposite number Patchell responded for the Blues a few minutes later but with the Scarlets leading 6-3 after 20 minutes, the game was yet to spark into life.
Welsh wingers Alex Cuthbert and George North provided the main go forward ball for their respective sides, Cuthbert flattening Andy Fenby with one carry but neither side came close to creating any clear cut chances.
The Blues did at least try something different, full back Jason Tovey chipping over the opposition defence inside his own half. Wing Harry Robinson collected but was adjudged to have had a foot in touch.
Thomas then missed an easy chance to extend the Scarlets’ lead, pushing a simple penalty effort wide of the right hand upright. And in a five minute period which summed up a poor first half, Patchell then repeated the feat, missing an altogether more difficult kick following a huge drive from the Blues forwards.
The last kick of the half saw Thomas atone for his earlier error, successfully adding a third penalty to give his side a 9-3 half time lead they just about deserved.
If the first half failed to fire, a break did little to liven up either side as the game continued to be dominated by the boot. Both sides were falling foul of referee Ian Davies’ whistle as the second half remained scoreless and in truth, the game desperately lacked a creative spark.
When the game did entertain, the ball was spread wide and from such a move, Patchell reduced his side’s deficit to three points. The fly half fed Robinson on the left touchline and although the winger was tackled into touch by three defenders, play was pulled back for an earlier penalty.
Patchell made no mistake and with 20 minutes left, the game was in the balance.
Thomas then missed another chance to pull his side clear and Liam Williams saw his late try disallowed for a forward pass but it mattered little as the Scarlets hung on.
