Late goals by Agustin Medina and Josh McEachran put a somewhat flattering effect on the scoreline from Blues perspective.
In truth, Blues were soundly beaten and just didn't function as they have in recent weeks.
Moha Ramos came out of the game in credit as he was the far busier goalkeeper and helped keep the scoreline down.
Blues did have chances, but too often they were loose and easily unhinged by an eager and purposeful host team, whose frontline caused constant problems.
Blues were missing their internationals Geraldo Bajrami, Ryan Stirk and Caolan Boyd-Munce, as well as the suspended Jonathan Grounds. Josh Andrews was unavailable due to injury.
But any mitigating circumstances had to be offset as the side were uncharacteristically below par.
Blues had the first opening when goalkeeper Ben Pierce sliced a clearance, leaving his net unguarded, but Miguel Fernandez's hurried attempt to find the target was blocked.
Barnsley served notice of their capabilities when Clarke Odur played in Mamadou Thiam after a slip by Mitch Roberts, but Ramos made a low save.
In the 10th minute Barnsley went ahead when Sam Fielding booted back a clearance on the volley, straight down the middle, and Elliot Simoes wriggled into the area to fire past Ramos.
Thiam then fired low and hard across the box from a corner, after slipping into a good position on the right, before Blues might have levelled things up.
Off-balance, McEachran arrowed a lovely pass crossfield to Wes Harding, who exchanged passes with Medina before sliding the ball into Odin Bailey, who miscued his initial attempt from inside the area before being crowded out.
Callum Styles slotted wide of the post with just Ramos to beat after a clever reverse pass by Simoes opened up the Blues defence again.
And in the 33rd minute it was 2-0 when Jordan Barnett put Thiam in, to the left of the box, and although Ramos parried his thumped shot from close range, Odur followed in to head the rebound home.
In the 38th minute it got worse for Blues when Thiam played Styles on the overlap down the left, his pull back eluded everyone and there was Odur to finish sharply, into the top corner.
At the break Blues brought on Nico Gordon and Jayden Reid, the striker's first involvement of the season after injury since signing in the summer.
They switched the three at the back and began the second-half brightly.
From a McEachran corner on the left, the ball was drilled back to Bailey on the edge of the area; he took a touch before cracking a shot that rebounded off the crossbar.
Then, from another McEachran corner, Kane Thompson-Sommers got in between a clutch of bodies to stoop forward but unfortunately the chance was headed wide.
The initial flourish for Blues was firmly ended when Barnsley scored again, in the 57th minute. Simoes tried to burst through and Joe Redmond, with a last-ditch tackle, got a toe-end on the ball, but Thiam took it on and stuck his shot past Ramos.
In the 67th minute Barnsley sliced open the Blues defence again, with the dangerous Simoes taking a return pass from Thiam before finding the top corner with a powerful effort.
Ramos denied Thiam, who cut back to evade two sliding challenges after being set up by Styles, with a parrying stop before Blues late rally.
It was a nice move that created Blues first goal, in the 85th minute. Substitute Jack Concannon backheeled the ball to enable Reid to drive towards the byline on the left and send the ball across to the back post for Medina to tap in.
Then Concannon and Bailey moved play to the right of the area for Harding, whose low sidefooter was saved at the near post by Pierce.
As uncertainty gripped Barnsley for once, from the corner that was cleared Reid and Harding shifted the ball back out to McEachran and his inswinging delivery eluded the 'keeper and sailed in at the far post (89 minutes).
Blues: Ramos, Harding, Burke, Mitch Roberts, Redmond, McEachran, Thompson-Sommers (Concannon 65), Medina, Fernandez (Gordon ht), George (Reid ht), Bailey. Sub not used: Rouse.