Blues boss Steve Bruce was keen to add the England international defender to his squad last summer but Mills preferred instead to move to Middlesbrough on a season long loan deal.
With the season now over Mills has returned to Leeds but is sure to be allowed to leave the relegated, cash-strapped Yorkshire side because of his huge salary and Blues are reportedly back in contention for his signature.
The right back role at Blues proved a problem area for Bruce over the last season. Jeff Kenna played a handful of games there before he was dropped from the side and ultimately moved to Derby. Olivier Tebily was then handed the role although Damien Johnson was also used in the position in a number of games too.