After a dull first half in which the visitors dominated possession, a soft goal from Stephen Forster started the goal-rush just after half-time.
Davies then took centre-stage, grabbing four goals in 19 minutes to give Lee Clark's Blues an evening to forget and send the Tykes above their opponents in the table.
Barnsley, who had taken the lead in their previous two fixtures only to be pegged back, started brightly with a series of attacks which caused problems for the home defence.
Despite the away team's greater share of possession and superior play going forward, the first half-hour passed without either side having a shot on target.
However, Chris Dagnall did just that for the visitors on 34 minutes, stinging Jack Butland's palms with a long-range effort before latching onto a through-ball four minutes later only to be thwarted by the Birmingham goalkeeper again.
Aside from a handful of misplaced crosses and a speculative drives from Wade Elliott, Barnsley goalkeeper Ben Alnwick was a spectator for much of the first half as the Blues struggled to get into their stride.
The teams went in all square at the interval. Barnsley boss Keith Hill would have been the happier of the two managers by far, but his day was about to get a whole lot better.
Hill's men continued to dominate after the break and Forster made the pressure count on 50 minutes as he gave his side a deserved lead.
After a miscued clearance by Jonathan Spector gave the Tykes a cheap corner, the resulting set-piece eluded the whole of Birmingham's defence, allowing Forster to poke the ball in at the far post.
Things went from bad to worse for Clark's men four minutes later as Barnsley exposed gaping holes in the hosts' defence and Davies nudged the ball home from close range to put the Tykes two up.
In what was fast becoming a superb day at the office for Barnsley, Davies extended his team's lead with a tremendous solo effort. The Wales international received the ball on the edge of the area before beating two Birmingham defenders and firing the ball beyond a fully-stretched Butland.
Davies then claimed the matchball by sealing an 11-minute hat-trick with a strike from inside the area, before adding to Birmingham's misery by getting his head to a Chris Dagnall cross to put his side 5-0 up with 15 minutes to play.
Marlon Harewood came on for Davies in the 85th minute, the four-goal hero leaving the field to richly-deserved standing ovation from the Barnsley faithful.