An own goal from Harlee Dean gave Darren Ferguson’s team an early boost and they doubled their lead in the 35th minute from the penalty spot through Jonson Clarke-Harris.
Jorge Grant struck for Posh at the beginning of the second-half before a tough afternoon for Birmingham City was made worse by the sending off of Gary Gardner.
Despite these setbacks, Lee Bowyer’s players kept fighting until the end and fashioned a number of chances that, if taken, could have made for very different viewing at the Weston Homes Stadium.
Troy Deeney was handed his first start after making a duo of appearances as a substitute in one of two changes to the team that kicked off against Fulham. The other switch saw Gardner brought into the midfield after also entering from the bench on Wednesday evening.
Peterborough made a flying start in Cambridgeshire and took the lead after just a minute. Dan Bulter set Siriki Dembele free down the left channel and his low centre was inadvertently put into his own net by Dean as he attempted to clear.
Blues had small openings for a leveller through Marc Roberts and Jeremie Bela deliveries before Deeney had the visitor’s first major opportunity on the 20-minute mark. The frontman peeled away from both centre-backs but could not keep his header from looping over Dai Cornell’s goal.
A further aerial chance presented itself moments later as Tahith Chong crossed, only for Lukas Jutkiewicz’ flick to prove a comfortable collect for the Posh stopper.
At the other end, Peterborough could have hit again was it not for Matija Sarkic’s fine anticipation to not only come out and meet Oliver Norburn but force him wide of goal.
However, the Blues ‘keeper was powerless to what followed as the home side doubled their lead with 35 minutes played. Grant was adjudged to have been fouled by Ryan Woods as he poised to shoot inside the area, giving Clarke-Harris a chance from 12 yards which he took coolly.
In truth, Bowyer’s men were likely to have been grateful for the interval’s arrival, although they continued to offer a threat and Gardner's half-volley skidded narrowly wide on the cusp of half-time.
Half-time: Peterborough United 2-0 Blues
Blues opened the second period on the front foot and shortly after the restart both Deeney and Woods had efforts well blocked as the away team looked to respond swiftly.
Instead, the task at hand was made even greater just five minutes after the break. Dembele pulled the ball across from the left and Grant set himself before finishing into the bottom corner.
This failed to stop Blues in their attempts to mount a comeback and Chuks Aneke was straight into the action following his introduction, finding space in the area as he glanced a header wide.
But again the job of finding a route back into the match was doubled in difficulty as Gardner was given his marching orders for a challenge on Posh skipper Nathan Thompson in the 65th minute.
Bowyer's charges refused to wave the white flag and Aneke again went close as Blues continued to pose a threat, with just the final touch letting them down.
Grant’s goal so soon into the second-half eliminated the dangerous 2-0 scoreline that the Posh held and made a turnaround highly improbable.
Blues sit just outside the top ten and remain on 11 points from their eight Championship fixtures.
Peterborough United (4-2-3-1): David Cornell, Nathan Thompson (captain), Ronnie Edwards, Frankie Kent, Dan Butler, Jack Taylor (Conor Coventry 72), Oliver Norburn, Joe Ward, Jorge Grant (Sammie Szmodics 83), Siriki Dembele (Harrison Burrows 83, Jonson Clarke-Harris.
Substitutes not used: Will Blackmore, Josh Knight, Idris Kanu, Joe Tomlinson.
Booking: Kent 71, Butler 93.
Blues (3-5-2): Matija Sarkic, Marc Roberts (Dion Sanderson 46), Harlee Dean (Captain), Kristian Pedersen, Maxime Colin, Gary Gardner, Ryan Woods, Tahith Chong, Jeremie Bela (Ivan Sunjic 46), Lukas Jutkiewicz (Chuks Aneke 58), Troy Deeney.
Substitutes not used: Connal Trueman, George Friend, Riley McGree, Scott Hogan.
Booking: Woods 35, Gardner (sent off), 65, Aneke 74.
Referee: Jarred Gillett
Attendance: 11,988 (away 3,985)
Blues have a clear midweek whilst other teams compete in the Carabao Cup before welcoming Preston North End in seven days. The month is then rounded off with a Tuesday night trip to Queens Park Rangers.