The Wales centre crossed either side of the break in an impressive all round performance and Liam Williams put the result beyond doubt.
The victory sees the Scarlets leap-frog the Ospreys into fourth in the RaboDirect PRO12 table ahead of the final round of the season.
“It was certainly job done in terms of the result but certainly not the league,” said head coach Simon Easterby
“After the Ospreys loss last night the expectation was on us and maybe in the first half we didn’t deal with it.
“We have to do that against Treviso next because we are now in the driving seat for that fourth spot and we have to thrive upon that.
“Jon was class and the partnership with Scott Williams was excellent. They gave us good go forward.
“He came back from the Six Nations and kept his form up. He’s a quality player and person.
“Hopefully he can have a massive impact and take us to the play-offs.”
Leigh Halfpenny opened the scoring after the Scarlets gifted the visitors a penalty but the hosts soon surged in front as Wales centre Davies sliced through Gavin Evans.
Owen Williams converted for the Scarlets but he failed to stretch their lead with a penalty minutes later.
Halfpenny kept the Blues in touch after Lou Reed was held agonisingly short but Williams responded to bring the scores to 10-6 at the break.
The Scarlets pack continued to cause the Blues problems in the second-half and it laid the foundations for George North to rampage down the right.
He was tackled short but the ball was recycled and following several phases the ball was spread to Davies, who powered over.
Williams nailed the difficult conversion to stretch the Scarlets lead to 17-6 and that was his last action as Priestland entered the fray.
And Easterby’s men quickly put the result beyond doubt as full-back Williams pounced on some loose ball and sliced through the Blues ragged defence to score.
“We were in the arm wrestle until 60 minutes but fundamental errors cost us,” added Blues director of rugby Phil Davies.
“It’s been the story of our season. At times we put some good rugby together but made silly mistakes.
“In the second-half we made two cracking clean breaks but both ended with knock ons and there were other mistakes like missed kicks to touch.”
