Rovers snatched the lead on 72 minutes through Colin Kazim-Richards' scrappy effort after Wojciech Szczesny had parried Martin Olsson's shot.
"We had 11 international players on the pitch at the start of the game. We had a lot of the ball and [Blackburn] defended well. We had 16 shots at goal, they only had one shot at goal," Wenger said.
"We made a massive mistake on the goal and we were not good enough to win the game, it is as simple as that.
"Not one defeat is acceptable. You are in this job to win every single game."
The home fans vented their frustrations as boos rang out around the Emirates Stadium at the final whistle.
"You do not want your fans to be happy when you lose at home to Blackburn when the team has one shot at goal," Wenger said.
"It is very painful and very disappointing to lose a game like that. What is important now is to focus on the next one."