The goals were spread around the team with Mitch Hancox (pictured), Andy Shinnie, Liam Truslove and Shane Ferguson all notching one apiece.
The match also gave an opportunity for Jonathan Spector to get 60 minutes of competitive game time under his belt for the first time since sustaining a thigh injury back in October.
Richard Beale’s side made the perfect start to the game and found themselves 2-0 up within the first seven minutes.
Hancox scored the opener after playing a one-two with Koby Arthur before sliding the ball past Barnsley keeper Elliott Chadderton.
The second arrived just four minutes later when Barnsley failed to clear Callum Reilly’s cross and it dropped nicely to Shinnie just inside the box and the Scot swept the ball home.
Blues continued to press and Shinnie blasted an effort over the bar, before trialist Chris Sutherland was denied a certain goal by a fine Chadderton save.
Barnsley looked slightly livelier in the second period of the match and Reilly was forced to block Marcus Pedeson’s goal-bound shot on his own line.
Despite the windy and rainy conditions, Blues continued to play some neat passing football and were rewarded with a third goal on the 80-minute mark.
Ferguson’s cross found the head of half-time substitute Liam Truslove and he headed home via the post.
Just two minutes later Blues made it even more of a miserable afternoon for their South Yorkshire visitors with a fourth goal.
An excellent long ball down the wing from Reilly found Arthur, who beat his man before sending an inch perfect ball past the front of the goal, and Ferguson slid in at the far post to fire home from close-range.
Blues U21s: Trueman, Spector (Higgins, 60), Hancox (Kelly, 60), Webb, Bell, Blackett, Sutherland, Reilly, Arthur, Shinnie (Truslove h/t), Ferguson.