Blackwell makes a case for the defence
The Blades are looking to bring in a goalkeeper and defender following the arrival of midfielder Leon Britton from Swansea.
With Chris Morgan, Nick Montgomery and Richard Cresswell signing long-term deals and Johnny Ertl and Daniel Bogdanovic having also been recruited, Blackwell's priority now is a centre-back.
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Hide AdHe said: "If I can get at least a centre-half in by the time the lads return for pre-season I'll be reasonably happy because I can do the others after that.
"The key area to consider now is the defence and we have to have the capacity for players to come off the bench and contribute without weakening us.
"I'm talking to two or three defenders at the moment, one in Scotland, one involved in the World Cup and another in Europe."