After a closely fought 70 minutes, which saw the Magpies lead 2-1 thanks to goals from Nolberto Solano and Laurent Robert, Rovers were steamrollered by Sir Bobby Robson's men as they pushed forward for an equaliser.
Late goals from Jermaine Jenas and Craig Bellamy and an own goal from Vratislav Gresko gave the scoreline a lop-sided look at the final whistle.
But Blackburn had started brightly, with Dwight Yorke twice threatening Shay Given's goal, but it was United who got the first breakthrough.
Robert twisted and turned to shake off Martin Taylor and crossed for Solano to head home past Brad Friedel in the 24th minute.
The Magpies were given a scare on the half hour when Andy Griffin caught team-mate Kieron Dyer on the shin, but following treatment the England midfielder was able to continue.
The first chance of the second half fell to Robert who tested Friedel with a 25-yard free-kick.
And Rovers leveller came from another set-piece, this one in controversial circumstances.
Sub David Dunn was fouled and having received treatment he did not leave the field and then played his part in the free-kick which eventually saw Damien Duff lashing home with a left-foot shot from 20 yards leaving Given with no chance.
But within seven minutes United were back in front having taken advantage of another free-kick.
Garry Flitcroft fouled Alan Shearer 25 yards from goal and Robert found a way through the Blackburn wall with his fierce effort and into the far corner of the net.
Graeme Souness then brought off Yorke and replaced him with Andy Cole, sparking a row between himself and the Trinidad & Tobago striker on the touchline.
But as Blackburn pressed for an equaliser they left themselves exposed at the back and Jenas made it 3-1 with five minutes left on the clock.
Two minutes from time Dyer skipped into the box and Gresko bundled the ball into his own net from close range.
But it was not over for Blackburn as United tore into them in the dying seconds and Bellamy got the deserved goal in injury time from Hugo Viana's cross.