The Blades put in a woeful performance and were beaten by a 40th-minute Craig Mackail-Smith header, but the night was even more costly for Blackwell as he lost Ched Evans, Jamie Ward and Lee Williamson through injury.
Peterborough United manager Jim Gannon has said he will be looking for clarification over his role when he meets with owner Darragh MacAnthony.Gannon is currently on a short-term contract at London Road.
Gannon - who is only under contract until the end of the campaign - has yet to meet chairman Darragh MacAnthony in the flesh but will when the Irishman jets in from Spain for the game with Watford on Saturday.
Peterborough left-back Scott Griffiths is on the sidelines with an adductor muscle strain, so Tommy Rowe will fill in again.Record signing Ryan Bennett is still out with an ankle injury.
The game between Peterborough United and Sheffield United in the Championship on Tuesday, 9th March 2010 at 19:30 takes centre stage for FootyMad's preview analysis.