Ya its just a black backdrop when ever any character thinks.......
If u clearly observe u can c the presence of chain in his neck!!!!
Lol, nahh, there's nothing there except his collar bone..
In that part, what happens is this: Naruto says ''I think I got it!''
Shot changes to him, thinking, no longer in sage mode, WITHOUT his necklace.
Also, for some reason, he's drenched in sweat and panting, although two seconds ago he was fine. But the thing is, Naruto has one continuous thought throughout all 3 panels: ''I think I got it'' *thinking* But it's not perfect yet, I need to keep practicing in sage mode, and I have to make sure no one sees me...''
If it was a flashback, you think he would stop, or pause sometime in between, but he doesn't.
Then with the necklace and whatnot, it doesn't make sense to me...