The picture is pretty ambiguous. I'd say Sasuke just developed Haki!
Kidding aside, the hypothesis that he borrow Juugo's power seems more likely than having Kyuubi's power. I just hate the very idea of Kyuubi being splitted up and shared, like its not the Sharingan being shared by many non-Uchiha isnt bad enough.
Another hypothesis comes to mind: how about him being more evil on his own will. The dark shadow is just a illustration of that. We are reading manga by the way, pictures speak louder than words.
Killing time while waiting for the next chapter. OnePiece, that is.