The Blades lost their first home fixture of the 2010/11 npower Championship season with a 3-0 defeat to Neil Wrnock's Queens Park Rangers.
The game saw the return of former keeper Paddy Kenny who joined the Loftus Road club during the summer in a £750,000 transfer.
The visitors soon took advantage of lacklustre defending and had stormed into a three goal lead inside 25 minutes; Hogan Ephraim, Jamie Mackie and Adel Taarabt (penalty) found the net.
United came back without reply though at the other end the Blades saw two penalty appeals turned down whilst Kenny made several top notch saves.
As well as Blackwell the Blades will be without young defender Matthew Lowton who serves the last of his three game suspensions; he was handed a straight red card on the opening day of the campaign at Cardiff City (1-1).
Winger Mark Yeates is expected to be included in the side to face his former club; despite interest from United he signed for the North East club in July 2009 from Colchester United under then manager Gareth Southgate.
He made his debut against the Blades in a 0-0 draw at the Riverside but following Southgate's departure and in the appointment of Gordon Strachan he soon lost his place and in mid-January 2010 completed a £500,000 transfer to the Blades after just 11 (+11) appearances and two goals for Boro.
Blades Last Line-Up (14/08/2010 vs. Queens Park Rangers): Steve Simonsen, Johannes Ertl, Chris Morgan, Kyle Bartley, Andy Taylor, Jamie Ward, Leon Britton, Nick Montgomery, Stephen Quinn, Richard Cresswell, Ched Evans. Substitutes: Robert Kozluk, Daniel Bogdanovic, Jordan Chapell, Kingsley James, Mihkel Aksalu, Mark Yeates.
Middlesbrough have been the biggest spenders in the Championship so far over the summer months but as Nottingham Forest proved last season that money doesn't bring success although the City Ground club did complete a top six finish.
The Reds have brought in a host of talent; mainly from over north of the board with Kris Boyd (Glasgow Rangers) being their highlight capture in close season on a free transfer after refusing to sign a new contract with the Scottish Premier League side.
A number of players returned to action from international duty before they embarked on a trip to the Walkers Stadium last weekend.
Stephen McManus, Kris Boyd, Barry Robson and Kevin Thomson were all in the Scotland squad whilst Tarmo Kink featured for Estonia - they all missed the League Cup victory at Chesterfield due to their international commitments.
The Reds drew 0-0 with Leicester City in what was an entertaining match but face Sheffield United in front of the Sky television camera's without Tony McMahon (thigh) and new signing Andy Halliday (groin) are all absent with injury.
However midfielder Gary O'Neil (knee) returned from injury to come off the substitute's bench to feature against the Foxes last Saturday.
Reds Last Line-Up (14/08/2010 at Leicester City): Jason Steele, Justin Hoyte, Matthew Bates, David Wheater, Stephen McManus, Nicky Bailey, Luke Williams, Barry Robson, Kris Boyd, Scott McDonald. Substitutes: Gary O'Neil, Leroy Lita, Tarmo Kink, Lee Miller, Julio Arca, Sebastian Hines, Danny Coyne.