Paul Dickov smashed home a penalty and Craig Bellamy scored on his return from injury to increase the pressure on Birmingham boss Steve Bruce.
Dickov blasted home his spot-kick four minutes into the second half after being felled by Matthew Upson.
The striker should have put Blackburn ahead before the break after racing on to Shefki Kuqi's through ball, but City keeper Maik Tayor saved his weak shot.
Substitute Bellamy then ran onto Morten Gamst Pedersen's pass to score late on.
The goals punctuated what was otherwise a dreadful game.
A turgid first half saw neither keeper tested in the opening 40 minutes as both sides squandered possession with sloppy passing and poor control.
Rovers midfielder Robbie Savage's every touch was predictably booed by the Birmingham fans, who had little else to make any noise about.
The boredom was lifted by a surprisingly entertaining end to the half as first City's Nicky Butt deflected Jermaine Pennant's right-wing free-kick just wide with his hip.
Dickov then wasted a glorious chance to fire Rovers ahead after racing on to Kuqi's threaded pass - Stan Lazaridis tried to clear, but missed the ball, leaving Dickov one-on-one with Taylor.
But the diminutive striker lacked conviction and hit a weak, side-footed shot, which Taylor comfortably saved low to his left.
Within seconds, Butt was caught in possession just outside his own box and the ball broke to Dickov, but he blazed over from 20 yards.
Dickov made amends in the 49th minute, blasting his penalty into the middle of the net after he had gone down following a tussle with Upson just inside in the box.
City had their own claims for a penalty waved away after Heskey crashed to the floor following contact with Lucas Neill.
The introduction of Bellamy for Dickov just after the hour mark, lifted the home crowd, and he was quickly into the action, laying on a great chance for Kuqi, but he screwed his effort wide from a tight angle.
Butt came close to levelling for Birmingham with their only real effort on goal, but Brad Friedel reacted well to save at his near post after the midfielder turned sharply in the box and shot low.
And City were made to pay when Bellamy beat the offside trap to collect Michael Gray's pass and slide the ball under the advancing Taylor.
Blackburn boss Mark Hughes:
"I am delighted that Craig Bellamy has been able to make an impact.
"His goal showed exactly what he gives us and why we bought him - he plays off the shoulder of the defenders and he is always a threat.
"But from the moment Paul Dickov scored the penalty, we were comfortable and there was only going to be one winner."
Birmingham boss Steve Bruce:
"It was not a penalty - it was a pathetic decision and the referee's got it woefully wrong.
"If Matthew Upson is tugging Paul Dickov, then how has Dickov fallen forward?
"My concern is that we would not have been given it - we were harshly treated - it is probably the worst decision I can ever recall for a penalty."
Blackburn: Friedel, Neill, Khizanishvili, Nelsen (Todd 45), Gray, Bentley (Emerton 75), Savage, Tugay, Pedersen, Kuqi, Dickov (Bellamy 62).
Subs Not Used: Enckelman, Reid.
Goals: Dickov 49 pen, Bellamy 81.
Birmingham: Maik Taylor, Tebily (Kilkenny 75), Cunningham, Upson, Clapham, Pennant, Johnson, Butt, Lazaridis (Jarosik 70), Heskey, Forssell.
Subs Not Used: Martin Taylor, Pandiani, Vaesen.
Booked: Heskey, Cunningham, Pennant.
Ref: B Knight (Kent).