And since Black Zetsu was the embodiment of Kaguya's will, he was equivalent to half a Kāma, only without Kaguya's power!
Just like Madara and Shin earlier! And Obito in the original story-after sealing the Ten-Tails inside their bodies, none of them had turned into Kaguya, whether they succeeded or failed.
Instead, only after Black Zetsu had given Madara Uchiha a truly heart-piercing greeting had the Ten-Tails turned on Madara, transforming him directly into Kaguya!
It was as though the Ten-Tails' power and Black Zetsu represented Kaguya's power and will respectively. Neither alone could revive her. Only when Black Zetsu merged his own will-which was Kaguya's will-into the Ten-Tails could she return.
Only that explanation made sense of why, after Black Zetsu struck Madara Uchiha, Madara hadn't simply lost control of the Ten-Tails and allowed it to break free of the seal, but had instead turned directly into Kaguya!
"You're still lying! Haven't you noticed that Black Zetsu, the embodiment of your will, has vanished? Aren't you afraid I'll kill you outright and erase you from this world forever?" After going over everything in his mind, Hatake Ziyan felt that his theory might well be the truth! He raised his blade again and addressed Kaguya.
"What do you mean?" Kaguya still wore that innocent expression as she stared blankly at Hatake Ziyan.
With that pure, innocent look, Kaguya seemed genuinely unaware of what Hatake Ziyan was talking about.
"Black Zetsu! Supposedly the embodiment of your will, and your self-proclaimed son. He was the reason you broke free of the seal and revived. But I obliterated him when you returned!" Hatake Ziyan stared at Kaguya and spoke in an icy tone.
"I really don't know any Black Zetsu!" Kaguya insisted.
"Hmm?" Looking at Kaguya's expression, Hatake Ziyan was no longer sure