Despite having performed successfully under Blackwell in the close season last term he has not been able to sign a contract at the Lane as Blackwell says he will go down the line into the Academy products at United.
The 26 year old is now without a club after being released by the Royals. Blackwell said: "I have made the decision because I don't think we will be short of cover in that area. We have Derek Geary, Kyle Naughton, and one or two other products in the academy pipeline.
"If you have an Academy you would hope to get one or two through to the first team, and Kyle did so well in his loan spell over the border last term I would really like to take a look at him."
Naughton was a regular reserve player until his loan spell at Gretna and then returned back to the Blades. Halls who is without a club made six appearances for United.