Black Zetsu thought it over and suddenly realized this could work. In some respects, it might even speed things up. Although he did not mind waiting, if there was a way to hasten his progress, he would certainly take it.
"What you propose may well be a viable method. However, the Reincarnate must reach a certain level of strength before their power can awaken," Black Zetsu said.
"Uchiha Ken awakened his Mangekyō Sharingan several years ago. Is that still not enough for Indra's chakra to awaken?" Katsuya Uchiha asked.
"Well..." Black Zetsu pondered. For Indra's chakra to begin awakening, the Reincarnate first had to develop his Mangekyō Sharingan to a certain stage. Doing so, however, often cost him his sight. During the Warring States Period, the Reincarnate had been in no position to obtain Asura's power.
But now Katsuya Uchiha intended to fuse Indra's and Asura's powers himself, so there was no need to worry about whether the Reincarnate went blind. They only needed to ensure that Indra's power remained within the Reincarnate.
"Although I have not met the Reincarnate, in theory, someone who has possessed the Mangekyō Sharingan for several years should have developed its power to a fairly advanced stage," Black Zetsu said.
Constantly using the Mangekyō Sharingan was not the only way to develop it further. Uchiha Itachi's Susanoo had reached its third stage some time before he lost his sight, whereas Sasuke Uchiha went blind shortly after reaching that same stage. Frequency of use was what determined when the Mangekyō Sharingan lost its sight.
As long as Uchiha Ken had not wasted those years, he should have developed his Mangekyō Sharingan to the point where Indra's power could awaken.
As for Asura's Reincarnate, Black Zetsu could not be certain. He knew that Uchiha Ken had become