A David Luiz free-kick and Zlatan Ibrahimovic's penalty on the hour mark put PSG 2-0 up before Yoann Touzghar pulled one back for Lens.
Late goals from Blaise Matuidi and Javier Pastore ensured PSG leapfrogged Marseille and Lyon - who face Montpellier on Sunday - into top spot in Ligue 1 ahead of the resumption of a tie with Chelsea that is finely poised at 1-1 after the first leg.
Blanc told the club's official website: "There was a lot of chances. We have to be more effective in four days. Because we will have fewer opportunities."
Matuidi added: "It was important to take this first place, even if it's only temporary. This is also good preparation before the Champions League match on Wednesday against Chelsea.
"We will go to London with confidence. I think we can do better especially in terms of efficiency.
"Wednesday's match will be a very difficult match against a confident team. But we will go there to get the qualification."
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