Lee Clark's woes continued as former club Huddersfield inflicted more misery on the under-fire Birmingham boss.
Striker Jermaine Beckford netted the first-half winner when he swept in James Vaughan's cross to lift Town back into the Championship top six.
Clark, who was forced to dismiss reports of a player revolt this week, saw substitute Nikola Zigic dismissed as the Blues dropped to fourth bottom.
Blues' Nathan Redmond crashed a curling first-half effort against the post.
City were booed off the pitch as the home fans watched back-to-back league defeats at St Andrew's for the first time since March 2011. Huddersfield's win came after last month's heavy 5-0 defeat by Barnsley - the club's biggest home league defeat in 25 years.
But any hopes held by Clark, who applauded both sets of supporters before kick-off, of a positive reaction against the Terriers were dashed by Beckford's first goal for the West Yorkshire club since joining on loan from Leicester.
Adam Clayton found Vaughan in acres of space down the right and he crossed for Beckford, who eased ahead of Steven Caldwell to convert from close range.
Blues were in need of inspiration and Redmond tried to do just that with a sweet curling effort against the inside of the far post from 20 yards.
But their hopes of securing at least a point were hampered when they were reduced to 10 men with 16 minutes remaining as Zigic was shown a straight red card for a high challenge on Callum Woods.
Birmingham manager Lee Clark:
"Football fans are working-class people, they want to see someone having a right good go for their team, in terms of tackling, heading, competing.
"There were four of five players in the first half who never did that. They've got to stand up and be counted. Okay, I'm responsible but I can't keep saying that. Can they start taking responsibility?
"I'm getting the flak from supporters, no problem I can take that. But I'm not sitting in the dressing room telling them not to tackle, to pussyfoot around, telling them to mis-place passes.
"They can't just hide behind me all the time, they've got to come out fighting."
Huddersfield boss Simon Grayson:
"I think deservedly we got the win, getting any points in the Championship is never easy.
"We started the game very well, very brightly, pressurised Birmingham when we needed to, played in the right areas with purpose and created some good opportunities. I'm delighted from that point of view.
"The back end of the game when Birmingham had nothing to lose we were resilient and withstood some pressure. To come here and get a result is satisfying from my point of view after two tough previous defeats."