With dominance in the forward pack, at the set piece, and with relentless defence, the visitors' contained the Drua's pace and flair.
Overcoming a slow start, the Auckland side brushed aside a number of handling errors exacerbated by the hot and humid conditions to lead 20-7 at halftime.
First-five Barrett kicked a penalty to open the scoring after 25 minutes when neither side posted any points on the scoreboard.
Then Rieko Ioane opened the try-scoring for the Blues, before the Drua, supported by a vocal home crowd, hit back through No.8 Elia Canakaivata.