Bruce admitted to the Sunday Mercury that Birmingham were in the queue to sign Bentley when he left Arsenal but failed to act upon their initial interest.
And he accepts that Blues' loss was very much Blackburn's gain.
"I made a mistake there - we should have taken him," said Bruce, who admitted many clubs were put off by Bentley's disappointing loan spell at Norwich.
"We all had the opportunity and I think we all knew he had ability so fair play to Blackburn - they've got him.
"There are always one or two you regret and I knew about him from my connections.
"He's got exceptional ability and he's flourished at Blackburn so obviously it was the right move for him."