Malik Mothersille opened the scoring in the first half, before Harrison Burrows claimed a brace after the break to send the Posh to Wembley.
Neil Critchley’s side will need to put this defeat quickly to the back of their minds, in order to build on Saturday’s 2-1 victory over the same opponent in League One.
The visitors produced the first truly testing moment of the evening, with Richard O’Donnell forced into a save and James Husband making a block, before the ball was eventually fired into the side netting.
Blackpool’s keeper was soon called into action once again, as the 35-year-old made a great stop to deny Joel Randall as he ran through clean on goal following a poor bit of defending on the half way line from the Seasiders’ back three.
Another big opportunity for Peterborough soon followed, with space opening up for Ephron Mason-Clark on the left. As the forward went to release his shot, Marvin Ekpiteta made a crucial tackle to block the ball.
The deadlock was broken ahead of half time, as the Posh’s pressure paid off. The ball was chested down into the path of Mothersille just inside the box, allowing the 20-year-old to take a touch before smashing the ball past O’Donnell into the top right corner.
Blackpool’s chances didn’t really trouble Jed Steer during the opening 45 minutes of play, and it proved to be a similar story shortly after the restart, with a header from Jake Beesley lacking any power to trouble the Peterborough keeper, following a good cross from Jordan Lawrence-Gabriel. An aerial opportunity also came the way of Hayden Coulson, but the wing-back was unable to keep down his attempt after a ball into the box from Matty Virtue.
There were also a few opportunities for the visitors before they claimed their second, with one effort being denied by a fantastic clearance from Ekpiteta. The victory for Darren Ferguson’s side was eventually confirmed ahead of the final 10 minutes, courtesy of a penalty decision, allowing Harrison Burrows to convert from the spot.
Into stoppage time, the Posh captain rounded off the victory with another goal, with his shot finding its way into the back of the net with a kiss off the post.
Here’s how the Seasiders performed:
1. How did Blackpool perform?
The Seasiders took on Peterborough United. Photo: CameraSport - Dave Howarth
2. Richard O'Donnell- 6
Like throughout the EFL Trophy this season, Richard O'Donnell produced a number of important saves for the Seasiders. Photo: CameraSport - Dave Howarth
3. Callum Connolly- 4
Callum Connolly was back in the starting XI following a short period on the sidelines with a back problem. The 26-year-old's side of the back three was exploited too often in the first half. Photo: CameraSport - Dave Howarth
4. Marvin Ekpiteta- 6
Marvin Ekpiteta was the pick of the back three. Despite having a couple of shaky moments like his colleagues, he was also on hand with a couple of superb challenges to get the Seasiders out of trouble. Photo: CameraSport - Dave Howarth
5. James Husband- 4
It was a bit too manic from the Seasiders in defence, with last ditch tackles required too often. Photo: CameraSport - Dave Howarth
6. Jordan Lawrence-Gabriel- 5
Things didn't flow too often Jordan Lawrence-Gabriel. He did get a couple of good balls into the box, but they came to nothing. Photo: CameraSport - Dave Howarth