Roberts celebrates after netting Blackburn's second
Bottom side Watford took another step towards relegation as Blackburn cruised to a comfortable victory at Ewood Park.
The Hornets, whose fate would be sealed if they lose at home to Manchester City on Saturday, went behind to Christopher Samba's header on five minutes.
Jason Roberts added a second for the hosts before a 20th-minute header from Douglas Rinaldi gave Watford hope.
But Benni McCarthy sealed the win on the half hour with a diving header from David Bentley's cross.
Bidding to avoid a fourth straight Premiership defeat for the second time this season, Rovers came out firing against their fellow FA Cup semi-final losers.
And it took a timely interception from Watford's Jordan Stewart to deny Roberts a shot on goal as he burst clear after just three minutes.
The hosts' early pressure paid off, though, when the influential David Bentley's corner from the left was powered home by Samba, although the 6ft 5in striker did appear to push his marker as he jumped.
Rovers extended their lead when Bentley hooked the ball forward and Roberts showed great strength to bundle the ball past keeper Ben Foster.
It could have been 3-0 as Morten Gamst Pedersen had the ball in the back of the net on 16 minutes, but he was deemed offside after following up another firm strike by Roberts.
But Watford were thrown a lifeline when Hameur Bouazza was given room on the right wing after a tidy move and his cross to the far post was brilliantly headed home by Brazilian Douglas Rinaldi.
But Rovers' two-goal cushion was soon restored when Bentley jinked past two challenges on the right touchline and his superb cross was put away with a flying diving-header from McCarthy, the South African's 20th goal of the season.
To their credit, Watford dominated the early stages of the second half and almost reduced the deficit when Steven Kabba's drilled right-foot shot was tipped around the post by Brad Friedel.
But manager Aidy Boothroyd's evening was summed up when his earlier substitution, James Chambers, was himself forced off with a nasty injury on 49 minutes and replaced by Marlon King, himself just back from a long injury lay-off.
The visitors battled hard to get back into the game but Rovers closed the game out with man-of-the-match Roberts just failing to add a fourth in injury time.
The victory moved Mark Hughes' side into the top 10, keeping alive their slim hopes of Uefa Cup qualification.
Blackburn boss Mark Hughes:
"We still think there is an opportunity for us to get Uefa Cup qualification and with five games left we still have to play teams we think we can get points from.
"There was a lot of disappointment after losing in the FA Cup semi-final at the weekend but we were excellent tonight.
"Our attitude was good and we created lots of chances in their final third, but Watford did well after the break."
Watford boss Aidy Boothroyd:
"We're still in there but the games are running out but we will fight to the bitter end.
"We did well to get back into the game at 1-1 but after doing the hard work we switched off and were punished.
"It's disappointing as we defended badly and gave away some daft goals."
Blackburn: Friedel, Emerton, Samba, Nelsen, Warnock, Bentley, Dunn (Kerimoglu 77), Mokoena, Pedersen (Gallagher 75), Roberts, McCarthy (Derbyshire 80).
Subs Not Used: Brown, Henchoz.
Goals: Samba 7, Roberts 10, McCarthy 32.
Watford: Foster, Mariappa, Carlisle, DeMerit, Stewart, Smith (Chambers 28), Mahon, Francis, Rinaldi (Kabba 46), Priskin, Bouazza, Chambers (King 49).
Subs Not Used: Shittu, Bangura.
Goals: Rinaldi 21.
Ref: C Foy (Merseyside).