The Blades turned down a club-record bid from Doncaster last month for the 24-year-old, who scored 15 goals in 35 appearances for Rovers during a season-long loan spell at the Keepmoat Stadium.
Blackwell said: "You can't predict what will happen. Three weeks ago Paddy Kenny was here and I wasn't expecting him to go.
"There has been interest, but Billy has been away on honeymoon so we have been waiting until he gets back.
"If there are any offers they will be considered and dealt with by Trevor Birch in the usual way."
Both Leeds and Burnley have been linked with moves for Sharp, who has reportedly been valued at £1million by the Blades.