Rovers started on the front foot and Bradley Orr had a header deflected wide from Rochina’s cross.
As the half-hour approach Blackburn forward Kazim-Richards had a penalty appeal waved away by referee Carl Boyeson when Jean-Alain Fanchone appeared to pull him back. Moments later, Watford goalkeeper Manuel Almunia got behind a well-struck drive from Jason Lowe.
It could have been an even more frustrating first half for Rovers when Jason 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 Martin 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 Morten Gamst Pedersen’s cross but his header was straight at Manuel Almunia.
Rhodes’ shot struck Tommie Hoban on the arm and the referee pointed to the spot.
Kazim-Richards grabbed the ball from the Scotland international but his confidence was misplaced; the striker slipping as he sent a 76th-minute spot-kick against the crossbar.
But former Huddersfield striker Rhodes would not be denied his seventh goal of the season – a fifth for Blackburn, in the second minute of stoppage time.
Danny Murphy, on for Lowe, played a return pass for Kazim-Richards to tee up Ruben Rochina.
The Spanish playmaker’s shot was deflected into the path of Rhodes who displayed his poacher’s instinct to rifle home left-footed on the turn from 12 yards.
Following five games without a win, Rovers now have two in succession under caretaker manager Eric Black and remain fifth in the Championship.
Defeat sees Watford slip to 16th – four points above the relegation zone.