Spaniard Ruben Rochina handed Rovers a deserved early lead from the penalty spot after David Murphy felled Mauro Formica but, crucially, Birmingham pulled level almost immediately courtesy of Marlon King's ninth goal of the season from close range.
Blackburn were undoubtedly the dominant side from the off. Birmingham skipper Steven Caldwell denied Jordan Rhodes after Jack Butland had raced out of his area unnecessarily, before Rochina fired wide from 12 yards when well placed.
It was no real surprise when the hosts took the lead after 13 minutes. Full-back Murphy brought down Formica in the area and Rochina stepped up to send Butland the wrong way from the spot.
But Birmingham pulled level shortly after with their first real attack. Chris Burke's cross was met by a combination of former City defender Scott Dann and Wade Elliott and the ball fell to Nikola Zigic, whose effort came back off a post and King was on hand to tap home from two yards.
Dann came within inches of a late winner against his former employers but a point apiece was just reward.