Spring is to put pen to paper shortly and sign a 1-year loan spell with the Blades. Blackwell has spent plenty of time with Spring in his managerial career. He had him at Leeds United until he departed the Whites and met up at the Hatters.
Blackwell said: "I have spoken to Mick Harford at Luton and it looks as if we will be able to do something on a season long loan basis. Matt is a good midfield player who I know from Leeds and Luton. When I left Kenilworth Road he had scored something like seven goals in about five months and although some of them were penalties, he does score from open play as well.
"Financially, things are very difficult at Luton at the moment which is why we should be able to get him here. He's a terrific athlete, gets up and down the pitch and into the box and, when Luton had a lot of games against Premiership opposition in the Cup, he was outstanding.
"He was Watford's promotion winnings side but the style there didn't really suit him and I tried to sign him towards the end of last season."