West Brom defender Nat Phillips was at fault for the second successive derby as Birmingham won 1-0 at The Hawthorns.
Phillips, whose back pass let in Fer Lopez in the 1-0 defeat to Wolves last time out, gave away a 41st-minute penalty scored by August Priske, who he fouled.
Albion lost their 10-match unbeaten home league run, the first since James Morrison took charge in February.
In a lively first 20 minutes, a mis-hit lobbed back pass by Phil Neumann caused panic for Birmingham.
Goalkeeper James Beadle dashed well outside his box to beat Aune Heggebo to the loose ball and clear.
At the other end, Albion goalkeeper Max O'Leary made a superb reflex save to deny Jay Stansfield's sliding shot from point-blank range.
Albion broke at speed and Heggebo steered a shot against the foot of a post with Beadle beaten.
Jhon Solis got his footing wrong after Heggebo's poor clearing header from Alex Cochrane's cross, the Colombian midfielder shinning his effort from 12 yards.
Felix Horn Myhre should have put Albion ahead in the 28th minute, heading just over the crossbar with more than half of the goal to aim at.
Albion went close again when Callum Styles curled a low free-kick around the three-man wall and Beadle tipped the ball behind.
Blues took the lead from the spot after Phillips caught Priske in the air with a clumsy challenge from Neumann's deep cross.
Priske coolly side-footed the penalty high to O'Leary's left for his second Sky Bet Championship goal of the season.
Albion had O'Leary to thank for preventing a second goal with a superb reaction save to divert Neumann's fierce downward header over the bar just before half-time.
No fewer than four Albion players slid in to block Solis' shot from six yards after O'Leary got a hand to Cochrane's corner.
Blues substitute Patrick Roberts appealed for a penalty after falling following minimal contact from Chris Mepham, but the referee was unmoved.
Ousmane Diakite was not far away with a long-range angled drive as Albion tried to find an equaliser.
Cutting in from the left, centre-back Mepham was the unlikely figure dancing through Stansfield and Solis before forcing Beadle to smother.
Substitute Barney Stewart's glancing header was tipped over by the keeper from Jimmy-Jay Morgan's cross.
Stansfield curled straight at O'Leary after substitute Max Bird won the ball high up for Blues.
West Brom right-back Brayann Pereira sliced high and wide after a 40-yard run from inside his own half.
At the death, O'Leary's foot denied Birmingham substitute Demarai Gray at the near post.
West Brom's James Morrison:
"I was really disappointed. I didn't see enough intensity and determination from us in the first half.
"It was important for us to get to half-time but another goal conceded just before half-time knocked the stuffing out of us.
"I felt we were on top in the second half but not really threatening. It was probably the same old story of not creating enough."
On Nat Phillips: "I do back him because at times in the season he's been excellent.
"I understand he's made two errors but it was just a misjudgement, stepping up on the cross when he shouldn't have, I feel.
"When I pick a team, I pick it for good reasons. He's OK. It might have done (affected him) from the last game but he's a strong character.
"I think he was the only one in the first half who smashed into a couple of tackles.
"He plays at 100 per cent all of the time but he's just been punished for a couple of mistakes that ultimately decided the game."
On the penalty award: "I haven't watched it back yet, but I initially thought there wasn't enough contact."
Birmingham's Chris Davies:
"We probably found another level in terms of where we were away from home.
"We produced some really good bits of play. We made a good start - we had two or three good chances and their keeper made a couple of good saves before we scored.
"I don't think we let them into our box much in that second half.
"These games are for the fans - it's local pride. For Blues fans to come to the Hawthorns and win is extra special and they can enjoy their night.
"We've found a formula away from home. It took us until January last year to win three away games and we've already done it.
"You need a bit of football but you need fight too if you're going to be a good team.
"We've looked like we're up for the fight, then we've got quality to play."
