Ezequiel Lavezzi's first-half goal proved to be enough for PSG, who climbed within a point of league leaders Lyon.
Blanc praised his team's defending, but said overall he had expected a much better performance from PSG.
"We were expecting better," he said.
"We shouldn't forget that Rennes defended very well and played very well tonight. We should have been more dangerous in attack.
"The satisfaction was our defence, especially with an eye on the very big matches we have coming up.
"Building from this solid defensive foundation, we will get results in these big games."
PSG defender Thiago Silva talked up the importance of the victory, saying the three points – not the performance – were the key.
"We struggled, especially in the opening 20 minutes," he said.
"The most important thing was the victory, especially at this time in the season, and playing at home.
"We got three important points. No, we didn't win 5-0, but in the end, a win is a win and that's the main thing."
