A bad-tempered game at the Millennium Stadium saw the Blues edge out the Scarlets 17-13 in the first half of Judgement Day.
Cardiff Blues number eight Robin Copeland was shown a straight red card for stamping on the head of Liam Williams, who received two yellow cards and was sent off as a result.
Both sides headed into the game looking to make it a hat-trick of wins. The Blues had scored wins over Edinburgh and Ulster in recent weeks, while Scarlets had conquered Zebre and Connacht.
It took just four minutes for Cardiff to take the lead, as Alex Cuthbert capitalised on some soft defence to grab a try, though the conversion was missed.
Gareth Davies hit two penalties in quick succession to extend Cardiff's advantage, after Scarlets' Liam Williams had been dispatched to the sin-bin for a tackle in the air.
Then, after Jordan Williams missed with a penalty attempt, Scarlets found their way back into the game through Rhys Priestland, who scored following a break from Jake Ball.
But the conversion was missed, and Davies responded for the Blues with his third penalty of the afternoon, before Priestland responded in kind on the stroke of half-time to make it 14-8 at the break.
Williams and Copeland were sent from the field following an unsavoury stamping incident on 55 minutes, reducing both teams to 14 men, before Scarlets found a way back into the game on 71 minutes.
Ken Owens made a fantastic break to cross, but the conversion was missed, meaning it remained 14-13 to the Blues.
And though Scarlets searched desperately for a winning goal, it was Davies who hit a crucial penalty three minutes from time to round off a tight win for Cardiff.
Pens: G Davies 4
Red Card: Copeland
Tries: Priestland, Owens
Red Card: L Williams
Cardiff Blues: 15 Dan Fish, 14 Alex Cuthbert, 13 Cory Allen, 12 Dafydd Hewitt, 11 Chris Czekaj, 10 Gareth Davies, 9 Lewis Jones, 8 Robin Copeland, 7 Josh Navidi, 6 Macauley Cook, 5 Filo Paulo, 4 Bradley Davies, 3 Tau'fao Filise, 2 Matthew Rees, 1 Gethin Jenkins (c)
Replacements: 16 Rhys Williams, 17 Thomas Davies, 18 Scott Andrews, 19 Chris Dicomidis, 20 Ellis Jenkins, 21 Lloyd Williams, 22 Simon Humberstone, 23 Gavin Evans
Scarlets: 15 Liam Williams, 14 Kristian Phillips, 13 Jonathan Davies (c), 12 Gareth Maule, 11 Jordan Williams, 10 Rhys Priestland, 9 Gareth Davies, 8 Josh Turnbull, 7 John Barclay, 6 Aaron Shingler, 5 George Earle, 4 Jake Ball, 3 Samson Lee, 2 Ken Owens, 1 Phil John
Replacements: 16 Kirby Myhill, 17 Rob Evans, 18 Rhodri Jones, 19 Johan Snyman, 20 Sione Timani, 21 Rhodri Williams, 22 Steven Shingler, 23 Frazier Climo
Referee: Ian Davies (Wal)
Assistant referees: Sean Brickell, Simon Rees (both Wal)
TMO: Gareth Simmonds (Wal)