Blues, relegated to the Championship this month, have signed the 34-year-old Sheldon-born player on a two-year deal, subject to a medical, as a free agent after he turned down a one-year contract at Everton.
"My contract ran out at Everton and I was offered a new deal there but I felt it was time to move on," Carsley told bcfc.com. "I wanted a new challenge and I didn't necessarily want to go to another Premier League club and struggle with them.
"Being realistic, we finished fifth in the league with Everton so I wasn't going to go to any of the teams above them and I couldn't see the point of going to any of the teams below them. So I feel I've done the next best thing and joined a club that's building for the future and trying to get where Everton are.
"I'd seen the job that Alex McLeish had done at Rangers and Scotland, where he had performed miracles. Alex just came across to me as a winner and his visions for the club match my own."
The Republic of Ireland international insisted: "Even though I'm 34, I'm not looking to wind down. All together last season, I played over 50 games including internationals.
"I've left a club that's going to be playing in the UEFA Cup again next season so it's not something I've done lightly. Everyone knows how I feel about Everton. It was a fantastic club and I had a great time there.
"We finished fourth, fifth, sixth and seventh so I've been a part of cementing that club in the top six.
"I've now come for a fresh challenge."