Two charged down kicks proved the difference as the Blues gained a satisfying 18-8 win over the Dragons.
Blues: (10) 18 Tries: Smith, M Williams. Con: L Thomas. Pens: L Thomas 2.
Dragons: (8) 8 Try: Wyatt. Pen: Sweeney.
Gethin Jenkins got a hand to Ceri Sweeney's attempted clearance on the half-hour, allowing Richard Smith to gather the loose ball and score.
The Wales prop then repeated the trick late in the match, the benificiary this time being flanker Martyn Williams.
Lee Thomas added two penalties and a conversion, while the Dragons had a Gareth Wyatt try and Sweeney's penalty.
The Dragons drew first blood when Hal Luscombe cut a great line and sent Wyatt flying down the right wing to score in the corner, Sweeney failing to convert the difficult conversion.
Young fly-half Thomas replied for the Blues with a penalty soon after, but it was still the visitors who were in the ascendency.
Luscombe was denied a try for a marginal crossing call by referee Alain Rolland and it took some fierce Blues defence to keep the Dragons out.
Wales A centre Tristan Davies, a summer addition from Leeds Tykes, certainly delighted the home fans with a huge hit on Steve Jones that knocked the Dragons hooker back in his tracks.
The Blues broke the siege and began to produce some good attacking moments of their own, US Eagles skipper Kort Schubert carrying well from the back-row and Rhys Williams hitting gaps well from full-back.
Put on the back foot, the Dragons conceded a try on the half-hour mark when they made a mess of a defensive ruck on their own line.
An untidy pass left Sweeney with too little time to clear and his kick was charged down by Gethin Jenkins to allow Smith to claim the try, converted by Thomas.
Sweeney made some amends with a penalty soon after, but the Blues still held a 10-8 lead at the break.
Both sides struggled to create clear-cut scoring opportunities after half-time and there was only a second penalty kick from Thomas, stretching the Blues' slender lead, to trouble the scorers.
Dragons coach Chris Anderson, a World Cup winner in rugby league, could see his first competitive match in union slipping away and looked to his bench for a solution.
Wales scrum-half Gareth Cooper replaced Gareth Baber and his energetic arrival added the drive and direction the Dragons had been seeking.
But the visitors were undone by another charge-down from Jenkins, the Wales prop again doing a fine impression of a flanker to let in skipper Martyn Williams for the match-clinching try.
The missed conversion hardly mattered as the Blues fans celebrated victory over their city rivals from along the M4.
Blues: R Williams; J Vaughton, T Shanklin, T Davies, C Morgan; L Thomas, R Smith; G Jenkins, G Williams, B Evans, C Quinnell, R Sidoli, N Thomas, M Williams (capt), K Schubert.
Replacements: J Yapp, M Leaman, M Jones, D Jones, M Lewis, D Dewdney, N Macleod.
Dragons: K Morgan; G Wyatt, H Luscombe, J Bryant, N Brew; C Sweeney, G Baber; R Snow, S Jones, C Anthony, I Gough, M Owen, J Ringer, J Forster (capt), R Bryan.
Replacements: S Tuipulotu, G Cooper, A Black, J Richards, R Thomas, P Sidoli, R Beattie.
Referee: Alain Rolland (Ireland)