Dexter Blackstock opened the scoring for the visitors shortly before the half-hour mark and after Chris Burke (55) had levelled, the striker struck again in the 63rd minute to secure the win for Forest and end Blues' 15 game unbeaten run..
"The performance was disappointing but at 1-1 I thought we were going to get a result," the Blues boss said after the game.
"You always hold up your hands if a team come and play really well and deserve to win but generally I don't think, although it was not one of our better performances, we deserved to lose the game.
"We were not ourselves on a difficult pitch in the first half, we didn't get the ball down into some good passages of play.
"I got a response in the second and I am really disappointed to lose.
"Losing our unbeaten record is tough to take but you have to give Forest credit. It was a dogged performance."