Troy Deeney scored twice against struggling Rovers before Almen Abdi and Matthew Briggs sealed a comfortable win which leaves the Hornets four points adrift of second-placed Hull. The result also means Blackburn are just two points and two places above the relegation zone and in the thick of a fight for survival.
Huddersfield gave their hopes of staying in the Championship a huge shot in the arm as they were inspired to a 3-0 victory over Millwall thanks to Jermaine Beckford's timely double.
Beckford, making his 250th league appearance, struck a goal in each half to sandwich James Vaughan's neat finish early in the second period as the Terriers moved three points clear of the drop zone.
Elsewhere, Brighton consolidated their play-off place with a 6-1 thumping of Blackpool. Ashley Barnes scored a late brace after Will Buckley, Matthew Upson, Andrea Orlandi and David Lopez had all got on the scoresheet for Gus Poyet's side, with Blackpool's only respite Dean Hammond's 53rd-minute own goal.
Leicester lost momentum in their promotion push as they slipped out of the top six with a 2-2 draw against play-off rivals Crystal Palace. Andy King cancelled out Danny Gabbidon's first-half strike before Palace took the lead again in the 67th minute through Stephen Dobbie, while Chris Wood equalised five minutes later to share the points.
That result meant Bolton leapfrogged Leicester into the final play-off place thanks to their 2-1 win over Middlesbrough.
Wolves slipped into the bottom three to take Huddersfield's place after their 2-1 defeat at Charlton, while Peterborough remain above the relegation zone on goal difference alone after they were beaten 3-1 by Derby.
Elsewhere, Hayden Mullins' 71st-minute goal for Birmingham was enough to end new Leeds boss Brian McDermott's two-game winning run at his new club as they were beaten 1-0 at St Andrew's and Barnsley remain in the bottom three after they drew 0-0 against Nottingham Forest.
Sheffield Wednesday and Ipswich drew 1-1 in the other game, Leroy Lita cancelling out Jay Tabb's early goal for Ipswich.