Experienced midfielder Hayden Mullins joined Blues from Portsmouth in July 2012.
Mullins, who can also play as a defensive sweeper or right-back, put pen-to-paper on a two-year deal for an undisclosed fee.
He is a former England under-21 international who joined Pompey in 2009.
He spent the last few weeks of the 2011/12 campaign on loan at fellow Championship side Reading, where he made seven appearances for the Royals during their promotion run-in.
Prior to his time at Fratton Park, he spent six years at West Ham United, and before that he was with Crystal Palace for five years.
He was voted Portsmouth's player-of-the-season at the end of 2010/11 season and gained an FA Cup runners-up medal with Pompey in 2010.