Following a goalless first half at London Road, three second half goals from Olivier Kapo, Cameron Jerome and Seb Larsson gave Blues victory against Darren Ferguson's side.
Peterborough 0 Blues 3
Blues: Doyle, Kelly, Parnaby, Taylor, Jaidi, Larsson, Nafti, Kapo (McSheffrey 66), Muamba. O'Connor, Jerome (Forssell 66).
Blues' final pre-season game of the summer will be against Sheffield Wednesday at Hillsborough on Saturday.