Two goals in three first-half minutes helped Blackburn ease to victory and dented Birmingham’s play-off hopes.
Tony Watt scored his first league goal for Rovers with a smart turn and finish from Shane Duffy’s header.
The hosts doubled their lead when Danny Graham slotted in from keeper Jason Steele’s ball forward after City’s Clayton Donaldson was denied a penalty.
Birmingham improved in the second half, but Jonathan Grounds’ drive against the woodwork was as close as they came.
The win was Blackburn’s fourth consecutive home league win – the first time they have managed that for six years.
Defeat means Birmingham, who have slipped further away from the top six, have now won just once in the last 10 on the road, and have a solitary away victory in 2016.
Blackburn manager Paul Lambert: “Watt’s goal was a brilliant bit of skill. He’s a strong lad, I think he’s just got to get fitter, for himself as well.
“He does things that are unexpected and gets people off their backsides and his goal was very good.
“I think Danny Graham has been fantastic, him and Jordi (Gomez). The biggest compliment I can give those two, coming from a Premier League club, is they’ve fitted in brilliantly, and are playing ever so well.”
Birmingham manager Gary Rowett: “The most decisive 10 seconds of the match, if not the season in some ways.
“I’m going to have to temper my words very carefully, just because we’re managers and we live and die by the officials’ decisions sometimes.
“I just asked the referee the question: why on earth would he go down? He’s about to score. There’s no benefit to him going down whatsoever, and I think the referee felt as though he went down easily.”