Theory: Sasuke's dark shadow is revenge and not the Kyuubi
First, I'd like to begin by acknowledging other members of this forum who have reached a similar conclusion independently of me and presented their ideas before this thread such as Naruto1217, SSJ4Naruto, Zeke26, amongst the many others. I'd like to emphasize this because I feel it's important for threads when applicable. Thus, you could consider them as citations in a bibliography I suppose, so please add reputation to them if you add it here.
I would like to attempt to present these conclusions more formally by citations and statements from the manga itself.
First, the shadow around Sasuke has nothing to do with the Kyuubi; rather it is a demonstration of surrendering to one's internal self for some goal they are passionate about. Here that is killing Danzou and Danzou alone.
Second, This means that Sasuke is relegating his rational self behind his emotions leading him to be consumed by hatred, revenge, anger, etc. This was a rational decision, but once done, he does not have the full rational available to return to himself unless he is stopped somehow.
Here Sasuke warns the samurai to get out of his way because he's pissed off. This is likely due to his betrayal from Madara and Zetsu, who have made his plans for killing Danzou much more difficult to impossible. However, he still has no desire to kill them at this point, consistent with his previous attitude.
Naruto 460 page 12 | One Manga
However, the samurai naturally refuse to stand aside, which is another blockade to the most important thing to Sasuke (killing Danzou). This is the breaking point for Sasuke. Up to this point he has managed his emotions fairly well when he has a mission (he's not emo), this breaking point pushes Sasuke to an emotionally dominant state:
Naruto 460 page 14 | One Manga
This consumption is different than Naruto's when he went to 4-tails, though they are extremely similar visually. At the time Naruto was consumed with hatred of Orochimaru and the desperation to save Sasuke no matter what it took, so he allowed the Kyuubi to control him allowing himself to be consumed completely.
Naruto 293 page 05 | One Manga
Notice several differences between these two. First, and most importantly, is that the fist in Sasukes case is in a single hand that is open. Now, is it a hand? There are six fingers that seem spread equally apart. It seems more like a platform than a fist. I think the open hand/platform is extremely important. I will develop on this below.
(recall Naruto 460 page 14 | One Manga)
In Naruto's case the Kyuubil, a sentient entity with a drawn body, closes Naruto between his hands. Thus, Naruto was completely consumed literally and lost himself mentally and physically, becoming the 4-tails.
Naruto 293 page 06 | One Manga
Thus, Naruto became a beast controlled by another sentient entity, while Sasuke is only consumed by hatred and revenge that suppresses his rationale. In both cases however, the decision was a willing rational one.
3.) Coming back to the open fist/platform that Sasuke is on, whilst Naruto was within a closed fist of a sentient Bijou (He lost his self completely), the Kyuubi, Sasuke has caged up his true emotions for a long time. As a result I believe this manifestation of malice grew within him. That is all this shadow is, it is Sasuke breaking down internally. However, unlike the 4-tails, Sasuke is not fully enslaved to these emotions and may develop into a character like Nagato. A person consumed with hatred and revenge, but also able to be logical. As long as that fist remains open, Sasuke can still be saved, even if it's while he's dying.
The shadow is not an external being like the Kyuubi in Naruto. It is a visual illustration of Sasuke's loss of control to his inner emotions of hatred, malice, and revenge. While it is drawn similarly to Naruto's loss of self to the Kyuubi there are multiple distinctions including visual ones that both separate the two and help define Sasuke's internal self. Sasuke is lost to the darkness now, but he can still be saved.
Thanks, and reputation to those shoulders I stand on.