Birmingham goalkeeper Richard Kingson needed treatment
Blackburn eased past Birmingham and into the Carling Cup fourth round.
Rovers, who had dominated from the start, took the lead when David Bentley fired brilliantly into the top corner.
Matt Derbyshire made it 2-0 from the spot after he was fouled by Rafael Schmitz and Roque Santa Cruz's diving header sealed Blackburn's victory.
Birmingham had keeper Richard Kingson taken to hospital as a precaution after he was hurt in a collision with Rovers stopper Chris Samba and the post.
Blues boss Steve Bruce had shuffled his side completely by making 11 changes to the side that had held Liverpool 0-0 on Saturday.
The visitors never settled and Blackburn, despite their recent poor form, were never seriously troubled.
Rovers had plenty of chances to open the scoring but Derbyshire had two efforts cleared off the line while David Dunn and Jason Roberts went close.
But Bruce would have been more concerned by the injury Kingson suffered five minutes before half-time.
He tried to deal with a Bentley free-kick but, under pressure from Samba, he smashed head-first into a post.
Kingson gingerly got to his feet but collapsed and, after several minutes of treatment, he was stretchered off to be replaced by Colin Doyle.
Roberts should have broken the deadlock up early in the second half when he ran clear but he fired horribly wide and managed to injure himself in the process.
Birmingham at last carved out a chance through Gary O'Connor but his shot was blocked and Blackburn continued to press and they got their reward through Bentley after 65 minutes.
The England winger beat Damien Johnson before letting fly with a fierce shot that flew past Colin Doyle.
Derbyshire's penalty made sure of Rovers' progress before Santa Cruz headed home his side's third from Brett Emerton's cross.
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