Board Thread:General Discussion/@comment-12611839-20130718170314/@comment-31283382-20161224183848

Frankly, I think that the theory is true: in his final appearance, Old Man stated that he never wanted for Ichigo to become a Shinigami. As such, I believe he purposely unsealed Ichigo's full power, but later resealed it, due to fear of Ichigo becoming far too strong. Note that Isshin somehow knew of Saigo no Getsuga Tenshō, and yet if he ever used it, he would lose his Shinigami powers, and I don't think Soul Society would be kind enough to just give them back. Also, he would lose Engetsu; considering Ichigo's Zanpakutō was not a real Zanpakutō to begin with, but rather a meld of Old Man and Zangetsu, he managed to retain it. I do believe that Isshin once used Saigo no Getsuga Tenshō, but didn't lose his Shinigami powers. Also, note that Mugetsu is black; just like Zangetsu's own Getsuga Tenshō, and Old Man stated that he frequently used Zangetsu's powers to help Ichigo.