Blues had come back from going a goal behind early on to lead at half time 3-1. "We took a big gamble in bringing back David Dunn, Clinton Morrison and Darren Anderton, two of whom haven't played for nearly six weeks.
The home side fought back after the break however and Blues had to make do with a point from an eventual 3-3 draw.
"Any time you go away from home and score three goals you expect to win.
"Considering the defensive record we've got you'd have expected us to have gone on and won the game," Bruce said after the game at Ewood park.
"Before today we had the best defensive record in the league bar none so its very, very frustrating.
"Three of the back four had also been away on international and I'm not looking to make excuses but physically they are tired.
"For a neutral it must have been a great game to watch," he added.
"All-in-all though, especially with the defensive record that we've got, then I'd have expected us to see the game through.
"After going behind after three minutes away from home I'd probably have settled for a point but, after going behind, for the half-an-hour leading up to half time I thought we played some terrific football which carved Blackburn open.
"That's seven draws now and we could quite easily have won two or three of them which would now see us sitting comfortably in the top half of the table.
"We'll just keep plugging away and hope that we can keep our big players fit."
Blues had come back from going a goal behind early on to lead at half time 3-1.
"We took a big gamble in bringing back David Dunn, Clinton Morrison and Darren Anderton, two of whom haven't played for nearly six weeks.