The former France international felt his side were suffering from fatigue as it was, before Zlatan Ibrahimovic netted a last-gasp winner at the Stade de la Beaujoire
An additional 30 minutes appeared an inevitability when Olivier Vigneau cancelled out Zlatan Ibrahimovic's opener in the closing stages.
However, the Swedish striker netted a last-gasp winner to send PSG through to their fifth Coupe de la Ligue final, much to Blanc's delight.
"In the first half, Nantes refused to play," the World Cup winner told France 2.
"In the second half, they realised that they could not win without changing [their approach]. They played higher [up the field].
"My team were maybe a little tired but they avoided extra-time and that was very good for us."
PSG will now meet either Olympique Lyonnais or Troyes in the tournament decider.