The striker scored his 17th and 18th goals of the campaign within the space of nine second-half minutes while Almen Abdi and Matthew Briggs added further goals as the Hornets recorded a first win in four attempts, and consigned Rovers - who had Leon Best sent-off in stoppage-time - to a fifth consecutive away league defeat.
With two games to go, Steve Bruce's Tigers remain favourites for second place, but Watford's much superior goal difference means Hull will need to win at least one of their remaining fixtures to prevent a late charge from the Vicarage Road club, who have now mathematically secured at least a play-off spot.
Zola recalled Fernando Forestieri in place of top-scorer Matej Vydra, who has not found the net for the Hornets since February, and the Argentina-born forward played his part with a trio of assists.
Zola's men were denied an opener by a stunning double save from Grzegorz Sandomierski. Daniel Pudil's cross from the right was headed goalwards by Deeney but the Polish keeper got down well to parry and he reacted superbly to prevent the ball rebounding back into his net after an inadvertent touch from Scott Dann.
Watford did not have to wait long into the second period for the deadlock to be broken. Seven minutes after the interval Forestieri went past Todd Kane and fed strike-partner Deeney who skipped away from Gael Givet and arrowed a drive across Sandomierski into the bottom corner.
Forestieri then sent Deeney clear and the striker clinically slotted in his second of the game and opened the floodgates.
Forestieri created his third goal of the contest with a through-ball for Abdi who fired in his 11th goal of the campaign.
Substitute Briggs got on the act with 15 minutes left as he calmly slotted in from close range after Blackburn had failed to clear Abdi's corner.
The afternoon got worse for Blackburn as Best was sent-off in the last minute of the game for an off-the-ball tussle with Cassetti and will now miss their last three vital games.