Stephen Jones, who along with skipper Dwayne Peel returned to regional duty from Wales action, supplied 15 points with the boot.
The match was in the balance until the Scarlets scored 20 unanswered points in the space of 19 second-half minutes.
The Blues, who rested Wales Grand Slam heroes Gethin Jenkins, Tom Shanklin and Martyn Williams, had a slender 17-15 interval lead following an entertaining first half.
With a stiff breeze at their backs it was the visitors who took the lead after just five minutes when Wales wing Jamie Roberts went in at the right corner for a converted try after the Blues moved the ball quickly along the line.
But the Scarlets responded six minutes later when Iestyn Thomas crashed over for an unconverted try after a searching run from centre Regan King.
In a closely fought half Blues lock Scott Morgan opened up a nine-point lead going over from a catch and drive line-out, that after a fine kick to the corner by Nick MacLeod.
But again the Scarlets responded with a converted try by centre Gavin Evans, sent over by strong-running full-back Morgan Stoddart.
With Scarlets lock Adam Eustace sin-binned for ball killing at a ruck that allowed full-back Ben Blair to convert a penalty, but that was cancelled out by a penalty for Stephen Jones in front of the posts two minutes from half-time.
The Scarlets had a chance to take the lead for the first time three minutes after the break but Jones was off target.
The home side did eventually take the lead with a Jones penalty on 54 minutes and the advantage was improved with an interception try for skipper Dwayne Peel after a good Alix Popham tackle on Blair.
And after that it was all Scarlets. Stephen Jones kicked a long-range penalty and then Dafydd James ran in for the bonus point try following good work from Jones and Watkins.
I. Thomas, G. Evans, Peel, James. Cons: S. Jones 3. Pens: S. Jones 3.
J. Roberts, Morgan. Cons: Blair, Morgan. Pens: Blair.
L Stoddart, James, King, G. Evans, Brew, S. Jones, Peel, I. Thomas, Schwalger, Manu, Eustace, Reed, D. Jones, G. Thomas, Popham.
Watkins for Brew (54), Priestlend for S. Jones (79), Cattle for Peel (79), John for I. Thomas (39), Rees for Schwalger (52), MacLeod for Reed (52), Bater for D. Jones (79).
Blair, J. Roberts, J. Robinson, G. Thomas, Selley, Macleod, Rees, Yapp, R. Thomas, G. Powell, D. Jones, Morgan, Molitika, Sowden-Taylor, Rush.
Stcherbina for Selley (71), Flanagan for Macleod (52), Spice for Rees (52), G. Williams for R. Thomas (60), Filise for G. Powell (60), Lewis for Molitika (69), White for Sowden-Taylor (49).
Tim Hayes (WRFU).