The centre back joined the club on a three-and-a-half year deal for an undisclosed fee. He is a former England Under-21 international.
Davies began his senior career at Luton Town before he joined West Bromwich Albion and became their second-youngest captain.
His rock-solid performances saw him make the short move to Villa initially on a season long loan in August 2007, before he made the switch across to Aston permanent.
Immediately prior to his move to St. Andrew's, Davies spent three months on loan at Leicester City.
Davies soon established himself as key player at Blues and was named PFA's Championship Team of the Year for 2011–12.
In what was a turbulent 2012/13 season for the club, Davies remained a rock in the centre of defence and won Blues' Players' Player of the Year Award and Player of the Year Award at the end of the campaign.