The Parisian giants looked set to reclaim top spot for the first time since December when first-half goals from Zlatan Ibrahimovic and Ezequiel Lavezzi put them in command at the Parc des Princes on Saturday.
But injuries to Yohan Cabaye, Marquinhos, Serge Aurier and Lucas Moura rocked PSG and the home side were left two players down in the closing stages after making all three substitutions.
Caen made the most of their numerical advantage and stunned Blanc's side as Emiliano Sala reduced the arrears with two minutes to play before a stoppage-time free-kick from Herve Bazile snatched a point.
PSG coach Blanc now faces the prospect of being depleted for Tuesday's UEFA Champions League round of 16 first leg at home to Chelsea.
The point ensured PSG moved level with second-placed Marseille, who drew by the same scoreline against Reims on Friday, and a point behind leaders Lyon ahead of their clash at Lorient on Sunday.
Recent signing Mathieu Duhamel scored his first two goals for Evian to secure a 2-0 victory at fellow strugglers Lens.
Duhamel joined Pascal Dupraz's side in a loan move on transfer deadline day earlier this month and marked his second appearance for the club with a match-winning brace to enhance Evian's chances of avoiding relegation.
Victory moved them four points clear of Lens and just a point adrift of Lorient, who are placed just above the drop zone, and ensured Antoine Kombouare's men are without a win in seven games in all competitions.
Evian substitute Daniel Wass and Lens midfielder Ludovic Baal were sent off after clashing in stoppage time.
Aleksandar Pesic struck six minutes from time as Toulouse beat Rennes 2-1 to move out of the relegation zone. Ola Toivonen looked to have salvaged a point for the visitors when he cancelled out Oscar Trejo's strike, but Pesic had the final say.
Giovanni Sio and Floyd Ayite were on target as Bastia won 2-0 at Nantes to move up to 13th place, while Nice had to settle for a goalless draw at Nice despite Djibril Sidibe's dismissal late in the first half.
The clash between Monaco and Montpellier was postponed due to heavy rainfall in the principality.
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