Hi, here is my analysis from the last episode (460).
The dark figure is none other than Dark side Zetsu. Here are the evidence i've found:
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red: Look at the eyes, they are really similar. There is 2 in this image because its not physical Dark Side Zetsu, its his chakra (detected by Karin). I see Zetsu as 2 part (like the sound nin Sakun and Ukon) but with a major difference. When they split they dont look full (see the manga image with the yellow circle). While detected the chakra would look like a full human body.
blue: Those ?tails? would be the shape of Zetsu's flower. Some thinks its a hand, but hands doesnt have 6 fingers. Sasuke in the middle of the flower would symbolize the fact that he is possessed. I'm not into the ?dark chakra fuelled by all the hatred? because we never saw that in Naruto, and there was alot of hate fueled ninja during all the episodes (exemple: pain).
yellow: Still, its a chakra form, but i think Zetsu's teeth are similar as Suigetsu's, pointy teeth. In the blurry chakra form possessing Sasuke, i think its plausible to think that Dark Zetsu as that kind of grin.
We dont know Zetsu at all. No jutsu is known (beside the transport one), so im sure he can possess people just like my theory explained. Karin saw a dark colder chakra in Sasuke. Dark Zetsu looks like the worst badass, cold hearted, cannibalistic etc... Sasuke was never clearly evil before, so he could have had a normal chakra. He never killed when he wasnt forced to, but now with the addition of the possession by Zetsu is action are turning evil.
Why Zetsu would have to possess Sasuke ? To make him unforgivable. Sasuke would never killed an innocent (Danzo isnt innocent). Now that he looks like a cold blooded murderer (killing neutral foes), other country (and surely the Fire Country) will really want to stop him. Who will be left on is side, filthy Tobi-Madara, who will use him to sync with Gedo Mazo.
Tobi is talking with Naruto to make him loose time (to prevent the heart changing speech on Sasuke) and to let Sasuke fall deeper in the darkness in the eyes of everyone else. That or Tobi will try to make Naruto change is mind on Sasuke. But that won't happen because Naruto will never back down on is word (thats is ninja way !!!).
Then there is the cursed seal theory. I don't think it was anything to have with the dark chakra. The fact that Juggo seems to know what is happening is maybe because of the transplanted flesh on Sasuke. He can feel the dark chakra from Zetsu, but isnt possessed because he isnt engulfed in it.
Well for now i think its enough...
