Black and spiral Zetsu could be one entity

[sheldon156]

I believe Spiral zetsu is the case/body of black zetsu,which is the spirit/soul of the original body.According the the many,Black zetsu(BZ) was one of kaguya son.If Kaguya had another son then this mean Black zetsu suppose to look human like Hagaromo and Hamura.

Spoiler

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Both Spiral zetsu and Black zetsu have unique feature that seperate them from normal zetsu.We should be awared already that Hagaromo (senju+uchiha )and Hamura (Hyuga clan +uzamaki) had both yin ang yang chakra.ZSpiral zetsu probably represent the yin nature and Black zetsu may represent the yang.When both black zetsu and spiral zetsu merge,then they will be back to their original form like hagarmo and Hamura.In addition Spiral zetsu and Black zetsu are completely different physically from a normal zetsu ,there spiral zetsu must have some purpose or something special to kaguya