Former Essex Senior League striker Dwight Gayle scored a first-half hat-trick as Peterborough edged Blackburn.
Gayle opened the scoring after pouncing on a rebound from Danny Swanson's shot, then added a second with a header.
The 22-year-old scored his third in the 27th minute after Rovers keeper Jake Kean parried Lee Tomlin's shot.
Jordan Rhodes found space to fire home for Blackburn with 17 minutes left and David Jones scored from 20 yards in the last minute but there was no equaliser.
Two seasons ago, Gayle was playing for Stansted in the ninth tier of English football. He joined League Two Dagenham & Redbridge in 2011, but they immediately loaned him out to Bishop's Stortford in the Blue Square Bet North, where he scored 42 goals last season.
Fortunes reversed at Ewood Park
The first game between Blackburn and Peterborough this season was at London Road in November and ended up in a 4-1 win for Rovers
He only made his Football League debut in August for the Daggers before joining Posh in November, initially on loan.
The forward opened the scoring within 11 minutes at Ewood Park when Kean denied Swanson with a double save, Gayle smashing home the rebound.
DJ Campbell was denied an equaliser by Robert Olejnik and moments later it was 2-0 when Gayle headed home after defender Karim Rekik failed to deal with Nathaniel Mendez-Laing's cross.
And he had his hat-trick sewn up before the half-hour mark when Mendez-Laing cut the ball back for Tomlin to shoot and Kean's save landed with Gayle, who lifted the ball over the goalkeeper.
Rhodes missed a good chance to pull one back and Posh could have led 4-0 at half-time as Swanson drove just wide.
Rovers made three changes at half-time, bringing off two former England internationals in Danny Murphy and David Bentley.
The hosts were better in possession after the break though Posh might still have increased their lead but for Rekik's fine tackle to deny Mendez-Laing.
Michael Appleton's side finally broke through in the 73rd minute when Rhodes nutmegged Michael Bostwick before firing home.
Jason Lowe squandered a chance to reduce the deficit, and when debutant Jones, one of the half-time substitutes, did so from outside the box it was too late for a Rovers comeback.
Blackburn manager Michael Appleton:
"They [the players] should feel let down themselves. They should be disappointed in themselves but that's the issue. Are they disappointed? Are they genuinely gutted at the fact that they've come in at half-time 3-0 down?
"Does it hurt them that much? There's certainly a lot of players in that dressing room that are hurting at the minute but I'm not convinced it's all of them.
"That brings a problem and it'll go either one way or the other. They know exactly which way it'll go from my point of view because they've been told. I think if the rules allowed me to make six or seven subs, I would have done."
Peterborough manager Darren Ferguson on Dwight Gayle:
"He's a very good player. He's better than I thought he would be.
"He's technical, doesn't give the ball away, and makes clever runs. He's the real deal. He will score goals. I thought him and (Nathaniel) Mendez were a real threat today.
"I don't know if we should have been hanging on as it doesn't justify our performance in the first half, which was outstanding. To come here and be 3-0 up, and it could have been more. I thought my players were outstanding."
Live Text Commentary
Full Time The referee brings the game to a close.90:00 +4:05
Dwight Gayle decides to take a short corner.90:00 +3:29
Unfair challenge on Jack Payne by Markus Olsson results in a free kick. Michael Bostwick takes the direct free kick.90:00 +1:42 Goal scored
Goal! - David Jones - Blackburn 2 - 3 Peterboro Goal scored by David Jones from just outside the penalty box to the bottom right corner of the goal. Blackburn 2-3 Peterborough.89:29
Foul by Karim Rekik on Lee Tomlin, free kick awarded. Mark Little takes the free kick.87:37
DJ Campbell challenges Tommy Rowe unfairly and gives away a free kick. Direct free kick taken by Tommy Rowe.86:06
Jordan Rhodes takes a shot. Save by Robert Olejnik.84:02
Adam Henley takes a shot. Save by Robert Olejnik.80:47
DJ Campbell challenges Grant McCann unfairly and gives away a free kick. Grant McCann restarts play with the free kick.79:46
Inswinging corner taken left-footed by David Jones from the right by-line, clearance by Tommy Rowe.78:40
Effort from inside the area by Jason Lowe misses to the left of the target.77:30
Direct free kick taken by Grant McCann.77:30 Substitution
Substitution Danny Swanson leaves the field to be replaced by Joe Newell.77:30
Grant McCann fouled by David Jones, the ref awards a free kick.77:03
David Jones challenges Lee Tomlin unfairly and gives away a free kick. Free kick taken by Jack Payne.75:19 Substitution
Substitution Kane Ferdinand is brought on as a substitute for Nathaniel Mendez-Laing.72:24 Goal scored
Goal! - Jordan Rhodes - Blackburn 1 - 3 Peterboro Jordan Rhodes fires in a goal from just outside the area to the top right corner of the goal. Blackburn 1-3 Peterborough.69:39
Free kick crossed by David Jones, clearance made by Tommy Rowe.69:39 Booking
Booking Danny Swanson receives a yellow card for unsporting behaviour.69:30
Unfair challenge on Cameron Stewart by Danny Swanson results in a free kick.
Cameron Stewart takes a shot. Save made by Robert Olejnik.68:22
Nathaniel Mendez-Laing delivers the ball.65:50
Grant McCann restarts play with the free kick.65:50 Booking
Booking Scott Dann is given a yellow card.65:34
Scott Dann concedes a free kick for a foul on Danny Swanson.65:03
The ball is crossed by Jack Payne, save made by Jake Kean.62:10
Outswinging corner taken left-footed by David Jones.60:26
DJ Campbell gives away a free kick for an unfair challenge on Michael Bostwick. Robert Olejnik takes the free kick.59:06
Tommy Rowe challenges Cameron Stewart unfairly and gives away a free kick. Free kick taken by David Jones.57:45
The ball is sent over by Jason Lowe, Robert Olejnik makes a comfortable save.54:26
Grant McCann takes the direct free kick.54:26 Booking
Booking Booking for Jason Lowe for unsporting behaviour.54:21
Jason Lowe concedes a free kick for a foul on Danny Swanson.51:53
Foul by Gabriel Zakuani on Jordan Rhodes, free kick awarded. David Jones takes the free kick.51:23
Shot from outside the box by Lee Tomlin goes wide right of the target.47:50
Corner taken right-footed by Lee Tomlin from the left by-line, save by Jake Kean.47:34
Danny Swanson takes a shot. Save by Jake Kean.45:36
DJ Campbell produces a left-footed shot from deep inside the penalty box which goes wide of the left-hand upright.45:01
The referee gets the second half underway.45:01 Substitution
Substitution David Jones comes on in place of David Bentley.45:01 Substitution
Substitution Cameron Stewart on for Danny Murphy.45:01 Substitution
Substitution Bradley Orr leaves the field to be replaced by Adam Henley.45:00 +2:06 Half time
Half Time The whistle is blown to end the first half.45:00 +1:20
The ball is delivered by David Bentley, clearance by Gabriel Zakuani.41:30
The referee penalises Jordan Rhodes for handball. Free kick taken by Robert Olejnik.38:15
Effort from outside the penalty box by Danny Swanson goes wide right of the goal.37:30
Shot on goal by Jordan Rhodes from inside the box goes harmlessly over the target.37:10
Scott Dann takes a shot. Robert Olejnik makes a save.36:26
Free kick awarded for a foul by Gabriel Zakuani on Jordan Rhodes. Bradley Orr takes the direct free kick.35:15
Dwight Gayle fouled by Karim Rekik, the ref awards a free kick. Michael Bostwick takes the direct free kick.34:28
Nathaniel Mendez-Laing handles the ball and concedes a free kick. Scott Dann takes the free kick.32:41
Danny Swanson is adjudged to have handled the ball. Grant Hanley restarts play with the free kick.31:48
Nathaniel Mendez-Laing produces a cross.30:39
Jordan Rhodes takes a shot. Robert Olejnik makes a comfortable save.27:24
Lee Tomlin takes a shot. Jake Kean makes a save.26:07
Lee Tomlin provided the assist for the goal.26:07 Goal scored
Goal! - Dwight Gayle - Blackburn 0 - 3 Peterboro Dwight Gayle scores a goal from close range high into the middle of the goal. Blackburn 0-3 Peterborough.25:05
Jordan Rhodes challenges Grant McCann unfairly and gives away a free kick. Robert Olejnik takes the direct free kick.24:29
Free kick awarded for an unfair challenge on Danny Swanson by Jason Lowe. Grant McCann takes the direct free kick.22:36
The ball is delivered by David Bentley, Robert Olejnik makes a save.18:19
DJ Campbell produces a left-footed shot from just outside the penalty box that misses to the left of the target.14:45
Unfair challenge on Grant McCann by DJ Campbell results in a free kick. Grant McCann restarts play with the free kick.13:54
Free kick awarded for a foul by Jason Lowe on Jack Payne. Michael Bostwick takes the direct free kick.13:03
The assist for the goal came from Nathaniel Mendez-Laing.13:03 Goal scored
Goal! - Dwight Gayle - Blackburn 0 - 2 Peterboro Dwight Gayle scores a headed goal from inside the six-yard box. Blackburn 0-2 Peterborough.12:50
The ball is sent over by Nathaniel Mendez-Laing,11:07
Danny Swanson takes a shot. Save made by Jake Kean.10:02
Danny Swanson provided the assist for the goal.10:02 Goal scored
Goal! - Dwight Gayle - Blackburn 0 - 1 Peterboro Goal scored by Dwight Gayle from close range to the top left corner of the goal. Blackburn 0-1 Peterborough.8:46
Tommy Rowe sends in a cross.8:24
The ball is swung over by Markus Olsson, comfortable save by Robert Olejnik.7:40
Header from deep inside the area by Jordan Rhodes goes over the bar.4:48
Jordan Rhodes takes a shot. Gabriel Zakuani gets a block in.3:54
Centre by Markus Olsson, Michael Bostwick makes a clearance.2:29
Danny Swanson produces a left-footed shot from deep inside the six-yard box which goes wide of the left-hand upright.0:42
Grant McCann has an effort at goal from long distance that misses to the right of the goal.0:00
The referee gets the game underway.
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