The French champions continued their stuttering start to the Ligue 1 campaign at the Parc des Princes as they were held to a 1-1 draw by Monaco.
Lucas Moura looked set to cap an impressive week for PSG when he scored the opener after 71 minutes - with Blanc's men having beaten Barcelona 3-2 in the UEFA Champions League on Tuesday.
However, PSG had to settle for a sixth draw in nine Ligue 1 matches as Anthony Martial levelled in the latter stages and Blanc felt his side were too fearful of conceding.
"My team were so scared of conceding a goal that they ended up conceding a goal," he explained.
"We didn't do what it took to maintain our lead - in the final two or three minutes you need to be more vigilant and more professional in terms of maintaining possession.
"It's a scenario that unfortunately keeps repeating itself and I hope this was the last time."
PSG's slow start to their bid for a third consecutive Ligue 1 title has allowed Marseille to steal a march at the summit, with Marcelo Bielsa's side leading by five points from Bordeaux in second.
Blanc is not overly worried by the seven-point advantage Marseille hold over his team and suggested a reduced number of games for the leaders was an important factor.
"It's a shame to be dropping the number of points we have been dropping lately. Marseille deserve to be on top, they're collecting a huge number of points and they deserve them, but they're only playing once a week," he added.
"My team were very tired in the last 10 minutes, and unintentionally we dropped back, even though we must not forget that Monaco had a good second half.
"We weren't brilliant but could we really be after the huge amount of energy we expended [against Barcelona] on Tuesday?"