Carlton Morris brought the Tykes level with a shot that took a deflection and hit both posts before nestling in.
Story of the match
Barnsley created the first chances of the match on 15 minutes when Callum Sykes saw his goal bound effort stopped by Thomas Kaminski, with Romal Palmer firing over the upright moments later.
Rothwell nearly put Blackburn in front after playing a dazzling one-two with Harry Pickering, bursting into the box and hitting the base of the post with his shot.
The deadlock was broken in the 25th minute when John Buckley won the ball off of a Tykes player, fed it to Rothwell and he beat Johnson to score against his former club.
Brereton then got into the action, a last-ditch challenge from a Barnsley defender keeping the Chilean off the scoresheet and Collins kept out his next shot.
Against the run of play, the visitors were level, Morris seeing his effort come off a Blackburn player and both posts despite Kaminski's best efforts to keep it out.
Blackburn were quickly on top of the action in the second half, Buckley forcing Collins into a save before the hosts finally found a winner.
Brereton was played in down the left, took on Barnsley substitute Jasper Moon, switched the ball onto his left foot and beat Collins into the bottom corner from an acute angle.
The lead could have been bigger as substitute Sam Gallagher tested Collins before Brereton went just wide.
Man of the match: Ben Brereton
He's the main reason why Blackburn are pushing for promotion, scoring the match-winner and looking lively and dangerous throughout.