Obertan, 20, has undergone a medical at Old Trafford ahead of his proposed £3m transfer.
However, Blanc, who enjoyed a spell at United under Sir Alex Ferguson before becoming a manager in his own right at Bordeaux, confessed he was shocked United swooped for a player who still has much to learn.
Obertan is a France U21 international who caught the attention of Europe's biggest clubs with some dazzling displays on the international stage at junior level.
However, he failed to progress at Bordeaux as Blanc would have liked and was loaned to Lorient last season to gain valuable first-team experience.
This, though, was not a success either as Obertan scored just one goal in 15 appearances.
It looked as though the winger had lost out on the opportunity to join United, who have been monitoring him for the past three seasons.
But with Chelsea also known to be interested in the player, Ferguson moved swiftly, catching Blanc by surprise.
He said: "I was surprised, but it is an unexpected chance for him to play with one of the best clubs in the world.
"Manchester United have been tracking him for a long time. They certainly hope to advance him, something that Bordeaux and Lorient have failed to do.
"He has the potential, but he must overcome psychological and mental challenges so he can express his true value."
Once United announce the signing of Obertan, Ferguson will have an abundance of wide players in his squad.
Antonio Valencia has already joined from Wigan for £17m, while Nani, Ryan Giggs and Zoran Tosic are also competing for the wing berths.