Blackwell wants to keep the squad competitive for next season
Sheffield United have extended the contract of keeper Paddy Kenny, while deciding not to offer new deals to Danny Webber and Ugo Ehiogu.
Lee Hendrie, David Carney, Nicky Law and Zeyn S-Latef have been made available for transfer.
On-loan players Craig Beattie, Greg Halford, Arturo Lupoli, and John-Joe O'Toole will return to their clubs.
Keith Quinn, Jamie Annerson, Maxime Chanot and Ben Starosta have all been released by boss Kevin Blackwell.
The Blades manager has said he wants to make room for the eight new players he wants to bring to the squad to remain competitive next season.
"It's going to be red hot next season, there's no doubt. The three clubs that have come down are going to be fierce competitors. It's going to be a very tough league, but last year was tough and we finished third.
"It's up to me now to get some new players in. I know there's going to be seven or eight new players coming in, which is more than I would have liked, but that's the way it is."
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