The Hornets failed to catch fire throughout the contest and lost the game to a Ryoya Morishita strike just after half-time.
Creating little of note, Rovers were able to see out the latter stages fairly comfortably on a disappointing day for Paulo Pezzolano's side.
Watford made an energetic start, Rocco Vata the liveliest and appealing for a penalty when he went down in the box after five minutes.
Kwadwo Baah(Image: Watford FC)
Blackburn were the first to go close, though, when Morishita smashed the bar with a long-range swerving effort that had beaten Egil Selvik.
Selvik was called into action shortly afterwards when Todd Cantwell forced him into a save as Watford became increasingly scrappy.
Rovers continued to put pressure on but Mamadou Doumbia did miss a presentable chance four minutes before half-time, Kwadwo Baah picking him out but skewing a shot wide from the middle of the box.
Blackburn started the second half strongly, though, Morishita giving them the lead after a simple through ball caught the Watford defence napping.
Pezzolano responded by introducing Edo Kayembe and Nestory Irankunda, the latter squandering a presentable chance with his first touch after a smart set-piece routine.
A flurry of corners came to nothing as the Hornets tried to build some pressure.
But Rovers looked comfortable and Todd Cantwell fired over with 13 minutes to play as Watford laboured.
Irankunda saw a free-kick deflected behind by the wall before Hector Kyprianou headed over as Watford huffed and puffed to little avail.
It ended up being a straightforward final few minutes for the visitors, with their hosts not launching any kind of late assault on their goal.