The Parisians won 3-1 at Nice to reclaim top spot in Ligue 1 from Lyon, with Pastore firing a brace at the Allianz Riviera.
Pastore was released behind the defence late in the first half by a Lucas Moura pass, and he finished clinically in the bottom-right corner.
The Argentine scored a second in remarkable fashion, juggling the ball to himself inside the area before firing off a shot that was saved - but he powered home when the ball found its way to him yet again.
"Javier, he is just ... unpredictable," Blanc said. "This boy does things that only talented players like him can do.
"At the same time, he is able to do things that can't be rationally explained.
"He can sometimes put at risk the team, even if at the end of the day we won 3-1 and he scored two goals.
"So I can only congratulate him but when I must tell him off, I'll do it because he is someone who needs that."
Blanc said he told Pastore to stop sitting back at half-time, and the product was two more PSG goals to earn the three points.
"We didn't plan to attack more from the left. But the players there combined very well," he said.
"At end of the first time, I asked Javier to play higher up the pitch and to not stay too close to our defence.
"For me, he played too close from our defence in the first half. As a result, we had trouble creating chances.
"But he is a player who wants so much of the ball that he wants the others to always give him the ball even if he is only 20 metres from our goalkeeper. I would prefer him to be 20 metres from the opposite goal.
"As dangerous as it can often be in front of the opposite goal, he can also be a real danger in front of our goal.
"I let him know this because we need his technical quality to create space and that's what happened mainly on the left today where he has shown a very good understanding with Lucas.
"Lucas has a lot of stamina and he likes to receive the ball in the deep. Naturally, we do attack a lot from this side."
