The win was all the more remarkable given that the Scarlets trailed 15-0 after half an hour with the comeback and nature of the win a huge source of pride to head coach Kevin George.
"It was a great game and I'm delighted with the way we played," he said.
"It's a lot of hard work come to fruition.
"With the wind it was always going to be a game of two halves but the skill level was high and there was a lot of intent to play.
"The wind made it difficult but we came out in the second half and played the game mainly in the Blues half. After that we were on top."
Playing with the wind in the first half, an early penalty for Blues South from fly half Luke Fish was soon followed by an interception try from left winger James Walsh.
Fish converted and when Walsh flew in for his second score in the 26th minute, the Scarlets faced a 15 point deficit.
However, in an entertaining game where both sides were committed to playing free-flowing, attacking rugby, the Scarlets fought back admirably.
"We thought a 10 to 15 point deficit would have been about par and we always thought we would get our opportunities," added George.
"We had to score tries and the St Helens surface was great for running rugby.
"We spurned a number of kicks at goal to run the ball and luckily we got our reward."
Scrum half Harri Randall got the Scarlets back into it with full back Jack Evans adding the conversion. And when replacement Ryan Davies crossed for his side's second try, with Evans again converting, the Scarlets trailed by a single point.
The Scarlets had not been helped by the sin binning of flanker Shaun Edwards in the first half but when Cardiff's Liam Free was yellow carded after half time, the Scarlets were on top.
A penalty from Evans put them into the lead for the first time and the Blues failed to reply with outside centre Kieran Jones crossing for a late converted try to seal the win.
"It's great for the Scarlets and a lot of our players have progressed to the Welsh set up recently which shows what we're doing is working," said a clearly delighted George.
"A number of boys are involved with the U20 and U18 this weekend and hopefully some players from this team will progress to that level too."
Scarlets: Tries: Harri Randall, Ryan Davies, Kieran Jones; Cons: Jack Evans (3); Pens: Evans.
Blues: Tries: James Walsh (2); Cons: Luke Fish; Pens: Fish.