I have a thought that Black Zetsu is a part of Kaguya's mind and the Kaguya we know is just another part of Kaguya. We see Kaguya could fool Isshiki who is very smart (make him off guard), therefore I have a theory that Kaguya splitted a part of her mind to create Black Zetsu and Kaguya in Naruto just have 1/2 of her mind. This's why Kaguya in Naruto is so dumb.