Blades striker Sharp, 23, failed to hold down a regular place in Blackwell's starting line-up last season and has been linked with summer moves to Yorkshire rivals Leeds and Huddersfield.
Blackwell told BBC Radio Sheffield: "Everybody's got the same opportunity, they return pre-season with the same level playing field.
"You've got to fight for the right to be a Blade and players have got to work hard to be a Blade.
"Everybody else here has got an opportunity. They've got to work hard and try and put right the wrong of not getting into the Premier League."
Lifelong Blades fan Sharp was released by former manager Neil Warnock in 2005, but returned to his hometown club for a reported £2million fee in 2007 after firing 32 goals for Scunthorpe in their promotion-winning year.