The Ligue 1 champions were held to a 1-1 draw at the Stade de la Route de Lorient after a somewhat lacklustre showing on Saturday.
PSG went ahead shortly before the break as veteran centre-back Zoumana Camara marked a rare start with a scrappy opener.
Rennes equalised in bizarre circumstances 10 minutes after the restart when substitute Paul-Georges Ntep's low cross evaded Abdoulaye Doucoure and rolled past Salvatore Sirigu.
Blanc felt that Rennes got their tactics right and believes his side may have been guilty of being naive.
"Physically, it's true that we were not at our best form," Blanc told the club's official website.
"I think this game, we had a tough time scoring against a team [that plays as a] group.
"There was little space and we might have settled for the 1-0, we continued to play and took it to Rennes.
"It's our fault, we did not know how to keep the scoreline, sometimes you [need to] win matches 1-0.
"Rennes had the right operation, they take a point and we have not preserved this score."
PSG have now failed to win each of their three league away matches this term and Blanc believes it is imperative to defeat Lyon in their next league clash at Parc des Princes.
"This is the third draw [away from home]," he added. "We have to win at home now."
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