The Swansea-based side nullified a second-half fight-back from their Welsh rivals at the Millennium Stadium to seal a 23-16 victory that moved them into the final play-off spot ahead of the Scarlets on points difference.
But Rees cut a frustrated figure despite the victory, insisting their performance was riddled with mistakes and missed opportunities - something that could prove vital if they are made to rue not getting a bonus point.
And with league leaders Glasgow Warriors and Leinster still to play, Rees has warned his players they must improve if they have any chance of defending their RaboDirect PRO12 title.
"We are pleased to get the four points because we didn't underestimate the Blues challenge," Rees told Wales Online. "However, that doesn't take anything away from the fact we are disappointed with our performance.
"We set our own standards. We are asking the boys to get better every week and we didn't do that.
"There were some pleasing moments in the first half, but then we had barely any control in what we were doing the more the game went on it felt like.
"Our kicking game was erratic and structurally there were too many flaws. We know we have got a power base and our set-piece certainly can be imposing but we didn't hammer that home as a collective.
"It's really frustrating. The Blues fronted up and made things difficult for us and put us on the back foot for periods in the second half. But it's about us and we are disappointed.
"It's perhaps a reality check. We are going to have look at our game and get better."
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