Wales centre Cory Allen scored the only try of the game after he raced onto Gareth Anscombe’s pass.
And it was fly-half Anscombe who settled matters at Rodney Parade with the winning penalty 12 minutes from time.
But they were made to work hard by a Dragons team who came agonisingly close to clinching a double over their local rivals for the first time in 11 years.
Tom Prydie twice kicked them ahead but crucially missed a fourth effort on 62 minutes after wing Matthew Pewtner had a try ruled out for crossing.
That left the door open for Anscombe to grab a deserved victory after Blues dominated the second half.
Prydie started where he left off at the Arms Park with a third-minute penalty while Anscombe pulled his first attempt wide.
But the New Zealander was on-hand to hand Allen an easy run to the line after Sam Warburton made the yards up front.
Warburton was back in the team following a shoulder injury and enjoyed a tight tussle with former Wales Under-20s team-mate Nic Cudd.
Dragons’ back-row star Taulupe Faletau also emerged unscathed from his return from injury that will have pleased Wales coach Warren Gatland.
Anscombe missed the conversion but opened an 8-3 lead early in the second half with a drop goal.
Prydie clawed back three points before Jonathan Evans missed a scoring chance from a quickly-taken penalty. The scrum-half could have scored himself but opted to pass to Hallam Amos, only for his poor delivery to allow Richard Smith to bundle his opposite number into touch.Â
Pewtner barged over the line but the score was scrubbed out for obstruction, though Prydie kicked Dragons ahead again with a third penalty.
That galvanised the Blues into action with the likes of Gethin Jenkins providing plenty of possession for Anscombe and full-back Joaquin Tuculet to pull the strings.
Smith was held up short but Anscombe landed a fourth kick before Blues held out for a much-needed win.
