Since we are slingin bs about who's foot it could be here is another potential. Do you recall that one Hashirama clone from which all other Zetsu sprang?
What if two things happened. Spiral Zetsu enveloped Yamato and then this Hashirama master clone became, active and started walking around as well. Our understanding of Black Zetsu is that he consisted of dark material that Madara described as his will.
This darkness may be what Karon senses. Anyway, just food for thought.