A late Dean Dewdney try saw the Blues win the battle of the Welsh basement clubs at Cardiff Arms Park.
Blues (9) 14
Pens: Thomas (3)
Scarlets (10) 10
A first-half try from Barry Davies and five Gareth Bowen points against three Lee Thomas penalties saw the Scarlets to a 10-9 half-time lead.
The visitors' forwards Scott Quinnell and Ian Boobyer were sin-binned, and the Blues finally took advantage.
With 11 minutes left, Shaun James broke the Scarlets' line and the ball came to Dewdney who got the decisive try.
It may be early season, but the tension of a basement battle was evident as both sides appear locked in a battle for Wales' third and final Heineken Cup place for next year.
Bowen, seeking to make the Scarlets fly-half jersey his own, opened the scoring with a penalty and impressed with his control of the back line.
Thomas - playing at centre for the Blues in the absence of Rhys Williams, with Nick Macleod in at fly-half - brought the
scores level with a 16th-minute kick.
The visitors got the first try on 20 minutes from a line-out five metres from the Blues line.
The ball was spun wide through Bowen and Salesi Finau, the latter putting Davies in for a clear run to the corner.
Bowen converted, but Thomas pegged the Scarlets back with two penalties.
With the break approaching, Scarlets No 8 Quinnell was sent to the sin bin for a dangerous tackle on opposite number
Kort Schubert, and Boobyer was lucky not to follow him when some neat footwork left Nathan Budgett's head streaming with blood.
The Blues failed to take advantage of the numerical superiority, but on 59 minutes the Scarlets were down to 14 men again
when Boobyer was sin-binned for a blatant punch at a ruck.
Indiscipline finally cost the Scarlets dear
The home side laboured to exploit the situation, but on 69 minutes James burst clear in midfield.
Craig Quinnell took the ball on from the breakdown, before Richard Smith spun it to Dewdney who crossed wide on the left.
The Scarlets then had to watch Finau being slowly carried from the field in a neck brace following a heavy collision with
Schubert, and Scott Quinnell limped off soon afterwards.
Prop Jon Thiel went close to a try in the corner for the Scarlets after good approach work, but the visitors lacked sparkle
and faded to defeat in a low-quality encounter, their only consolation a bonus point for finishing within seven points.
"That meant everything to us, we've been working hard all season but it hasn't shown on the pitch," said Blues lock Craig Quinnell.
"We haven't lost at home in the league and knew we had to bounce back from last week's loss to Stade Francais.
"There weren't many chances against two good defences, but we played simple rugby and stuck to our discipline."
Blues: C Morgan; D Dewdney, S James, L Thomas, F Tuilagi; N Macleod, R Smith; J Yapp, R Thomas, M Jones, C Quinnell, R Sidoli, N Budgett, R Sowden-Taylor, K Schubert.
Replacements: B Evans, A Lewis, G Williams, D Jones, N Thomas, Roberts, M Nuthall.
Scarlets: B Davies; G Evans, M Watkins (capt), T Selley, S Finau; G Bowen, Rhodri Jones; P John, M Rees, J Thiel, C Wyatt, A Jones, G Thomas, I Boobyer, S Quinnell.
Replacements: D Maddocks, C Hawkins, B Griffiths, J Mills, C McDonald, A Thomas, M Taylor.
Referee: Alain Rolland (Ireland)