Aizen's deal with Uchiha Madara was comprehensive, but both sides held something back.
Aizen had always maintained a disciple-like respect for this legend of the Ninja World. After all, those who excelled were teachers. Even if he did not understand something like Chakra, it was only natural to show some respect to a Senior in that field.
As for Uchiha Madara, to put it nicely, he responded to kindness but not force. To put it bluntly, he was gullible and easily swayed. Though he clearly knew that Aizen scoffed at his ideals, Aizen had always been respectful and had exchanged information and tools with him, so Madara in turn treated Aizen with considerable respect.
Although using Kagami Uchiha as a messenger was somewhat troublesome, and tested the adaptability of an old man like Uchiha Madara, who already had one foot in the grave, Madara did not trust Aizen's abilities for a certain reason—without a doubt, he suspected Aizen would beat up an old man, though Aizen had no such intention. Their communications had therefore never involved meeting face-to-face; they learned about and described things to one another indirectly.
This exchange was no different. Aizen gave Uchiha Madara development experience and theories concerning Bakudō within the Soul Reaper system, and after some thought, Uchiha Madara decided to show off something big.
It was absolutely not because Uchiha Madara felt somewhat uncomfortable after seeing Aizen's seemingly cheat-like rate of development. He was merely demonstrating the abilities of a seasoned Senior.
Then, Uchiha Madara nearly died in the process of creating Black Zetsu, but fortunately, the result was fairly good. Black Zetsu was successfully born.
This half-black, half-white lifeform claimed to be a fragment split from Uchiha Madara's soul. It possessed a great deal of research material and knew many secrets. It could