The Blues defied the conditions with some fine tries
Blues: (17) 29
Tries: Pen, James, Morgan, Czekaj
Cons: Blair (3)
Scarlets: (0) 10
Con: C Thomas
Pen: C Thomas
The Blues made light of a quagmire pitch at a packed Cardiff Arms Park to claim a bonus-point derby win.
Nicky Robinson set up a dazzling opening try that he was denied scoring as Ceiron Thomas dragged him back - a penalty try resulted, Thomas binned.
A move from the Blues' own 22 ended with a Tom James try, and after the break Scott Morgan crossed from short range and Chris Czekaj got the fourth.
A late penalty try from a collapsed scrum was the Scarlets' only joy.
The victory sees the Blues climb above the Llanelli side in the Magners League after a miserable Christmas period for the Scarlets that has seen a postponed game with the Dragons and a hammering at the Ospreys.
It was really tough in those conditions, but both sides went out there and tried to play rugby
Man-of-the-match Scott Morgan
The Arms Park game was itself in doubt until the opening whistle, incessant rain in Cardiff leaving the south side of the pitch resembling a muddy bog.
Scarlets flanker Dafydd Jones conceded an early penalty - kicked by Ben Blair - and was then yellow-carded for killing the ball on the floor.
The visitors held out when they were down to 14 men, but soon after Jones' return Blues outside-half Robinson made a looping break from deep in his own half.
He grubber-kicked past the cover defence and went for the line, but was held back by opposite number Thomas.
Czekaj followed up and grounded the ball, but referee Hugh Watkins awarded a penalty try instead and sent Thomas to the sin bin.
Bravely defying the conditions, the Cardiff side spun the ball across their three-quarters from near their own line and found Jamie Robinson in space.
He carried forward before finding rookie wing James, who cut inside to cross under the posts for his first Blues try at the Arms Park.
Wales rivals Phillips and Peel square up in Cardiff
Thomas got the Scarlets' first points of the game with a penalty shortly after the break, but any thoughts of a comeback were soon snuffed out.
The home side booted a kickable penalty to the corner, and from the driving maul man-of-the-match Morgan ploughed over the line.
Morgan - touted by Wales legend Phil Bennett for an international call-up - was then sin-binned for killing the ball in a rare spell of Scarlets pressure.
The 14-man Blues collapsed a scrum to give the visitors their penalty try, but then went looking for the bonus point.
The ball came to Nicky Robinson after a ruck, and a beautifully floated pass from the outside-half freed Czekaj for a flop-in at the corner.
The home side's day was marred slightly in the closing minutes as Bradley Davies limped off injured and then John Yapp collapsed with a leg injury in the deep mud.
Man-of-the-match Scott Morgan:
"It was really tough in those conditions, but both sides went out there and tried to play rugby.
"We have to be really pleased with that win and with the performance."
Blues: Blair, James, J. Robinson, Stcherbina, Czekaj, N. Robinson, Phillips, Jenkins, Thomas, G. Powell, Jones, Davies, Morgan, Shellard, Rush.
Replacements: Macleod for J. Robinson (64), Yapp for Jenkins (71), Goodfield for Thomas (71), Goode for Jones (69), White for Shellard (57). Not Used: Fairhurst, Luveitasau.
Scarlets: G. Evans, James, King, Watkins, M. Jones, C. Thomas, Peel, I. Thomas, Rees, Manu, Cooper, MacLeod, D. Jones, Bater, N. Thomas.
Replacements: Daniel for James (53), G. Evans for Watkins (41), G. Thomas for Bater (41). Not Used: Dunlea, Owen, A. Jones, Stuart-Smith.
Referee: Hugh Watkins (WRU)