The match will take place on Saturday July 9 (kick-off 3.00pm) and mark the sterling work undertaken by long-serving club secretary Blackwell over his 38 years with the Pilgrims.
New Posh boss Grant McCann will take charge of the Sky Bet League 1 promotion hopefuls for the first time since his permanent appointment as manager.
All match arrangements will be announced in due course.
Boston United would like to thank Barry Fry and the Peterborough United management for their help in arranging this testimonial fixture.