The 24-year-old, previously at Blackpool, will formally join on July 1 after his current deal with the Tangerines expires.
The agreement with the Wales international comes fresh off the back of similar deals for forwards Lee Novak and Andrew Shinnie and goalkeeper Darren Randolph.
Llandudno-born Eardley began his career at Oldham Athletic where he made over 100 appearances before moving to Blackpool for an undisclosed fee in August 2009. He quickly became a mainstay in the Seasiders team that had a one-season stint in the Barclays Premier League during the 2010/11 campaign.
Eardley, who bagged a stunning strike against Blues during the 2-2 draw at Bloomfield Road in November 2011, has 16 senior caps to his name having previously represented his nation at U17, U19 and U21 level.
The thoughts of Blues’ latest recruit can be read here