Davies nods a brace as youth delivers for Blues.
The centre-back nodded the visitors in front amid pouring rain in the 35th minute at Oakwell and sealed the win in the 77th minute - his fourth header in converted in six games.
Birmingham manager Clark named five teenagers in his starting line-up and praised their performances as they helped the side to a first win in four.
He said: "The last two games we've had, the four points we've taken and the manner that we've done it in is great credit to the young boys - that was quite an old team for us now; five teenagers in the starting 11.
"What they're doing is showing great spirit and enthusiasm."
Davies scored either side of a Jonathan Greening goal to secure the win.
Barnsley had, however, started the better and missed plenty of chances early on.
Craig Davies - a boyhood Blues fan who scored four goals in September's 5-0 rout at St Andrew's - was among the culprits, blazing over a second-half penalty after Steven Caldwell's handball. And the hosts were punished almost immediately as Curtis Davies, unmarked from another set-piece, converted in the 77th minute.
It was only a second win in nine games for Lee Clark's 19th-placed side and just their third away victory of the season.
Barnsley enjoyed the better of proceedings early on, Jim O'Brien seeing an effort blocked by Curtis Davies although the visitors did force a succession of three corners in which they failed to prosper.
Birmingham's young England goalkeeper Jack Butland kept the score down at St Andrew's in September and he was twice called upon here in the opening exchanges.
First he got down low to deny Stephen Dawson before a better save kept out Marcus Tudgay's header.
Luke Steele was still to be called upon as the half-hour mark passed, with Barnsley much the stronger.
But Curtis Davies - as he did in Saturday's 2-2 draw against Burnley - headed Birmingham in front.
Robert Hall delivered another excellent set-piece, Birmingham's main threat, which Davies nodded home unchallenged at the near post.
However, the lead did not last long as Birmingham's frailties in defence were exposed.
Barnsley applied pressure almost straight from kick-off with Dawson seeing a strike blocked and, with the visitors struggling to fully clear their lines, the ball broke for Greening, who converted low from distance.
The soaking wet playing surface was hardly helping matters but Barnsley continued to enjoy the better of matters in the first half, despite Blues enjoying another three successive corners.
The visitors' defence was exposed again within a minute of the restart but a swift move ended when Chris Dagnall could only slide the ball behind.
Chris Burke was introduced off the bench for Birmingham and he added a fresh attacking dimension.
John Stones saw an effort blocked by a late Blues tackle and, with the game flowing from end to end, Barnsley were awarded a spot-kick in the 73rd minute for a somewhat harsh handball decision against Caldwell.
But Craig Davies, who had only been on the pitch five minutes, blazed his spot-kick high over the bar.
A superb stop from Butland kept out a header from fellow sub Matty Done before Curtis Davies then settled matters in the 77th minute via another inchperfect delivery from Hall.
The result left Blues 19th in the table, on 29 points, but Clark - who has been under pressure in recent weeks - was full of praise for his young team.
That was quite an old team for us now; five teenagers in the starting 11 ''
Birmingham's Curtis Davies rises high to head home Birmingham's second and winning goal
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|Publication:||The Birmingham Post (England)|
|Date:||Dec 27, 2012|
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