First, Dragons' full-back, Phil Dollman, dropped a high ball without an opponent in sight.
The Dragons full-back did, however, score soon after although Bosc again missed the conversion.
It was the kicking of the Dragons' full-back, Tom Prydie, with 13 points, that proved the difference between the sides.
Dragons full-back Josh Dugan and backrower Trent Merrin, who are both likely to be included in Laurie Daley's NSW side named on Tuesday, had quiet games from the Dragons but made it through unscathed.
Two Halfpenny penalties extended the Blues' advantage but then an unlucky bounce from a Patchell chip gave the Dragons full-back and man of the match Dan Evans the chance to break clear before putting the wing Harries in for the try.
Dragons full-back Evans had a glorious 40 minutes.
Catalan Dragons full-back Clint Greenshields is to leave the club at the end of the season.
Bristol Bears have signed Dragons full-back Zane Kirchner as a short-term injury replacement for Charles Piutau.
Catalan Dragons full-back Brent Webb was twice sinbinned at the Halliwell Jones as Warrington ended the visitors' winning start to the new season.
While Roberts' knee was being treated on the field before his substitution, Dragons full-back Martyn Thomas collided with him and the medical staff.
Two of Super League's top try scorers, Dragons full-back Morgan Escare and Saints winger Tommy Makinson, who both have 14 tries to their name this season, were up against each other.
