Why?
Because after semi retiring for a few years, he rejected any proposal when asked to come in the middle of the year.
Except people have forgotten about him and he took a huge hit when he was rejected by Lyon in favor of Garcia last summer.
And to be honest, it has cost Lyon HUGE time since then.
So Blanc is thinking that if he manages Barca the way he wants for a year and is successful doing so, he'll
1/ be able to stay in a top 5 club
2/ won't stay because the new board wants their guy but big clubs will have noticed.
Let's keep in mind that with Blanc, PSG never lost a domestic competition (a feat never done before or after, by PSG or any other club). They lost twice on away goals to Barca and Chelsea in the CL. They only real CL defeat was that loss to City right after the whole Aurier/Ibra fiasco and clearly the team didn't respond mentally after that.
Even with Neymar and Mbappe, no PSG team has come close to what Blanc did with his PSG teams.
When he managed Bordeaux, he had a bunch of nobodys (Chamakh was his best striker) and Gourcuff and made them play amazing football, steamrolling everyone in L1 to the title (including Lyon which was a powerhouse then) and topping its CL group with Juventus and Bayern.
I have no idea why Blanc is so underrated in football circles. It's not as if he wasn't a legend as player also.
My only question mark with him is that all he did was with JL Gasset as assistant coach and Gasset is now his own manager since they split (on good terms BTW). Blanc has never worked without him.