The Blues, shrugging off driving rain, fashioned a sensational seventh-minute score.
Blues flanker Martyn Williams put his skipper and namesake Rhys into attacking mode from a retreating scrum. Chris Czekaj then made headway down the right flank before sending scrum-half Mike Phillips over unopposed to climax a stunning 89-metre raid. Nicky Robinson duly added the conversion.
The in-form Blues struck a further blow after just 12 minutes, with fly-half Robinson on target with a long-range penalty.
The Dragons were thrown a lifeline minutes later when Blues’ hooker Rhys Thomas found Michael Owen at a lineout, and the back-row giant drove over for an opportunist touchdown, which Craig Warlow converted.
A snap drop goal by Robinson added three more points to the Blues tally before the Dragons responded through a Warlow penalty.
The Dragons were reduced to 14 men either side of half-time with Owen in the sin-bin, but the Blues failed to add to a 13-10 lead during his absence.
Instead, the visitors had Wales lock Robert Sidoli banished to the bin for taking out rival Luke Charteris at a lineout - but crucially the Blues took their cue to grab an important score.
Patient build-up by the Cardiff forwards created an overlap for Craig Morgan to send fellow wing Czekaj over for an unconverted try.
A Warlow penalty narrowed the deficit to five points but the Blues have developed a winning edge of late and comfortably closed down the game.
: Tries: Owen. Cons: Warlow. Pens: Warlow 2.
: A. Thomas, Wyatt, Bryant, Sweeney, Brew, Warlow, Baber, Black, Brown, R. Thomas, Gough, Charteris, Ringer, Owen, Forster.
: Breeze for Warlow (71), Ireland for Baber (67), Sidoli for Gough (69), Hall for Ringer (67).
: Tries: M. Phillips, Czekaj. Cons: N. Robinson. Pens: N. Robinson. Drop Goals: N. Robinson.
: R. Williams, Czekaj, Stcherbina, T. Davies, Morgan, N. Robinson, M. Phillips, Yapp, R. Thomas, Jenkins, D. Jones, Sidoli, Schubert, Rush, M. Williams.
: L. Thomas for T. Davies (76).
: Tim Hayes (WRFU).