Jordan Rhodes' injury-time winner spared Colin Kazim-Richards' blushes as Blackburn claimed a second 1-0 home win this week.
Having spent the majority of the game on the defensive, Watford were set to leave Ewood Park with the point that appeared to be the limit of their ambitions.
Kazim-Richards slipped as he sent a 76th-minute spot-kick against the crossbar after denying Rhodes the opportunity to step up himself.
But the former Huddersfield striker was not to be denied his seventh goal of the season - a fifth for Blackburn - in the second minute of stoppage time.
Following five games without a win, Rovers now have two in succession under caretaker manager Eric Black and remain fifth in the npower Championship.
Defeat sees Watford slip to 16th - four points above the relegation zone.
Blackburn's line-up displayed two changes to the XI that beat Sheffield Wednesday in midweek, with Ruben Rochina in for calf-injury victim David Dunn and Morten Gamst Pedersen restored in favour of Marcus Olsson.
Gianfranco Zola changed his Watford front two, Troy Deeney and Fernando Forestieri coming in for Alex Geijo and Matej Vydra, while Connor Smith made a first senior start in central midfield and Jean-Alain Fanchone was handed a debut a left wing-back.
Rovers started on the front foot and Bradley Orr had a header deflected wide from Rochina's cross.
It proved to be a rare opening as the hosts struggled to break down Watford's five-man defence.
As the half-hour approach Blackburn forward Kazim-Richards had a penalty appeal waved away by referee Carl Boyeson when Fanchone appeared to pull him back. Moments later, Watford goalkeeper Manuel Almunia got behind a well-struck drive from Jason Lowe.
Kazim-Richards fired a shot narrowly past after being played in by a cute pass from Rochina, but Zola's men reached the interval with their goal intact.
It could have been an even more frustrating first half for Rovers when Lowe carelessly coughed up possession outside his own box but Forestieri's effort was deflected behind.
During a quiet opening to the second period Blackburn lost defender Gael Givet to a foot injury, meaning Olsson was introduced at left-back.
In the 65th minute Rovers enjoyed a clear opening when Kazim-Richards stole in front of the Watford defence to meet Pedersen's cross but his header was straight at Almunia.
Rhodes then headed over from Olsson's cross and Rochina fired over before Blackburn appeared to pass up their chance for the win.
Rhodes' shot struck Tommie Hoban on the arm and Boyeson pointed to the spot.
Kazim-Richards grabbed the ball from the Scotland international but his confidence was misplaced.
Lowe's departure with a suspected hamstring injury looked set to cap a frustrating afternoon for the hosts before Rhodes' dramatic final word.
Danny Murphy, on for Lowe, played a return pass for Kazim-Richards to tee up Rochina.
The Spanish playmaker's shot was deflected into the path of Rhodes who displayed supreme poachers' instincts to rifle home left-footed on the turn from 12 yards.