Hornets manager Gianfranco Zola recalled Fernando Forestieri in place of top-scorer Matej Vydra, who has not scored since February.
However, it was Blackburn who fashioned the first good chance after Watford keeper Manuel Almunia picked up Lloyd Doyley's pass back and conceded an indirect free-kick inside the area. But the opening came to nothing - Lee Williamson setting up Scott Dann, only for the defender to blaze over.
Watford then took control and it did not take long after the interval for the hosts to make the first breakthrough - Forestieri feeding strike partner Deeney who arrowed a drive into the bottom corner.
Nine minutes later the scoreline was doubled with Forestieri again setting Deeney free to slot home past Blackburn stopper Grzegorz Sandomierski.
Forestieri created his and the game's third goal with a throughball for Abdi who fired home before, with 15 minutes left, substitute Briggs calmly slotted in after Blackburn had failed to clear a corner.
Rovers's misery was complete when Best was shown a red card in the 90th minute for an off-the-ball incident, meaning he will now miss his side's last three games of the season.
Watford manager Gianfranco Zola:
"What can I say, everything went perfect - a lot of goals, good performance, a good quality of football and a big crowd and a Sardinian day out there. It's been an ideal Saturday afternoon.
"When you have all these performances on the pitch then of course the result is more likely to be on your side.
"It is a crazy Championship, it is unbelievable at the top and at the bottom and I think it is going to be like this until the end. I'm expecting changes in the situation until the end."
Blackburn caretaker manager Gary Bowyer:
"I thought we did well first half, the first goal was always going to be crucial after the backlog of games. I'm disappointed in the manner we conceded but you've got to give credit to the lad who took his goals well.
"It was probably a big game too many for us today but we've got to regroup quickly, we've remained positive and calm throughout and that's exactly the message we've said to them again.
"We've always known we had a game in hand, we always said it might come to goal difference, we've just got to go game by game again but at 0-0 I couldn't see that coming in the second half."
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