It's just more like Hypothesis, and I highly doubt that Kishimoto will give another chance to Black Zetsu in “The Last” movie as was given in series. However, I would like to discuss about it.
Here’s my theory: Ok, I was wondering that White Zetsu is made of Hashirama’s DNA, So Black Zetsu should be made of something, even if Black Zetsu is Kaguya's will, I’m just curious of what is made of I think BZ is made of Truth-Seeking Ball. Pardon me, I'm just assuming, and this is my theory, I have no knowledge of what BZ is made of, I have no proof. It just occurred to me that IT has shape shifting abilities and more, and in this Wiki it was mentioned IT is made of black mass, and I assume probably IT was part of Kaguya's body, so maybe she had given life to her broken part (her Ten-Tails part might had cut off prior to sealing) and the Truth-Seeking Ball (Black chakra, malleable) or possibly Yin chakra merging with her body parts or tailed beast created an entity known as Black Zetsu. This might have happened during the creation of all things. I would encourage if someone came up with different theory.
Some similarities: BZ mentioned itself as Kaguya's child and likes to be attatched to Kaguya as it was seen hiding in her kimono.Not to mention it even opened up new dimension in its last attempt to save his mother, same technique as Kaguya’s. I don’t think that a normal thing could Impale Madara who became Ten-tails Jinchuriki so it is obvious that BZ was made of TSB. Also BZ can shift shape but also likes to remain in humanoid form.
Well I'm confused, I think the Chakra natures' Yin-Yang, Nature energy TSB, Tailed beast, White zetsu are all linked related somehow. You might have already known and discussed about this, but I’m highlighting this in discussion because it’s seems somehow related to BZ and this is for everyone’s opinion.
So the Point of my Discussion-Is Black Zetsu made up of anything like Yin Chakra/Black chakra and White Zetsu Yang Chakra? I'm suggesting this could lead to something in the future. So we know it is locked in genesis dimension, so it’s not important, but who knows it might come back. Or may be Tonneri was also manipulated by Black Zetsu in some other way we probably don’t know If only the BZ history was clear and this was my guess
Kishimoto stated that he had decided to make Zetsu half black and half white to "better show his split personality". Naruto series always intrigues me that there is always something related and connected, and Black and White Zetsu fusion resembles Yin Yang symbol (as I have always know which is symbol of black and white forming circular pattern).
As Kaguya’s Expansive Truth-Seeking Ball had all Chakra natures and also Black Zetsu’s nature is similar to that, I see no reason why my theory can’t be considered. Correct me If I'm wrong anywhere.