Black Zetsu became kaguya's will when it bonded to Obito...?

[KameRasenGumGumGetsuga]

This is the only reasonable explanation. Obito still had the juubi's shell inside of him and he still had the rinnegan so when black zetsu tried to take him over , kaguya's sealed chakra inside of the shell must have sprouted and took over black zetsu and Obito slowly. In no other time can it be examined that black zetsu was kaguya's will...

Kaguya's chakra was most likely still inside of the juubi a little bit. This is a very audible reason

 

[Sukix]

.. madara extracted the gedo statue from obito -> juubi´s shell

 

[ObitoGokuKakashi]

Nice explanation.

Spoiler

This once again shows how great Obitos will is.

 

[SnakeGermany]

I think Madara had a part from Kaguya in him from his birth (Since he's a clan member from Kaguyas grandson) and this part got extractet when he created BZ but Kaguya just waited for the right time.

 

[conradoserpa]

No, it has always been like that.

Just look at Spiral Zetsu leaving Yamato's body on his own

 

[VongolaX]

Obito..

Even in seemingly death, you neve seize to amaze us.