Okay before you all go saying What 
??? and every other thing, first read then comment. It is something I think everyone missed out this week. Note that the reference translation is that of ***********, as I think they rarely mess up.
Now let's begin.
First of all take a look at the image below and read what it says.
Notice the way Black zetsu, that is on Naruto, says that as Kaguya looks at them, she is reminded of her US, HER TWO SONS. Quite fascinating, don't you think? Black Zetsu isn't calling himself as her third child, but calling theirselves as her two children. He is talking in the sense of plural.
In this panel below
Notice how it again says Sealed in US.
Also the below panel
What is most fascinating about it is that Sasuke doesn't seems to be surprised at all until he asked that question and got the reply and then this happen
The moment BZ answered that question Sasuke got surprised by the change of tone and behavior. This is where the true BZ emerged. He stopped acting as well. But at the same time, he said Kaguya as "HIS" mother, and this time not "US"
And finally this panel where the dialogue seems different for both halves.
Now here is what I think. Black Zetsu are not just one entity but 2. He is the ideal image of Hagoromo and Hamura, the way Kaguya wanted her children to be. Obedient and whatnot. Thus the reason why he is spouting US instead of ME.
CONCLUSION-
??? and every other thing, first read then comment. It is something I think everyone missed out this week. Note that the reference translation is that of ***********, as I think they rarely mess up.Now let's begin.
First of all take a look at the image below and read what it says.
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Notice the way Black zetsu, that is on Naruto, says that as Kaguya looks at them, she is reminded of her US, HER TWO SONS. Quite fascinating, don't you think? Black Zetsu isn't calling himself as her third child, but calling theirselves as her two children. He is talking in the sense of plural.
In this panel below
You must be registered for see images
Notice how it again says Sealed in US.
Also the below panel
You must be registered for see images
What is most fascinating about it is that Sasuke doesn't seems to be surprised at all until he asked that question and got the reply and then this happen
You must be registered for see images
The moment BZ answered that question Sasuke got surprised by the change of tone and behavior. This is where the true BZ emerged. He stopped acting as well. But at the same time, he said Kaguya as "HIS" mother, and this time not "US"
And finally this panel where the dialogue seems different for both halves.
You must be registered for see images
Now here is what I think. Black Zetsu are not just one entity but 2. He is the ideal image of Hagoromo and Hamura, the way Kaguya wanted her children to be. Obedient and whatnot. Thus the reason why he is spouting US instead of ME.
CONCLUSION-
Black Zetsu is not just one entity but two. He is the ideal image of Hagoromo and Hamura born from Kaguya's will at the time of her getting sealed. That's it.
