Board Thread:Naruto Discussions/@comment-1872728-20160329033053/@comment-734582-20160430143443

Ravenlot 27 wrote: Well, I'm not sure that any Sharingan evolution can restore lost eyesight. At least, this is the case of EMS > Rinnegan evolution. If the condition to restore eyesight to a Mangekyo Sharingan is that it needs to swap owners, the Rinnegan evolution can simply be explained that way.

After all, Madara awoke Rinnegan because he recreated Hagoromo's chakra, which can also be interpreted as his chakra changing to Hagoromos, that is, a change in person. So the Rinnegan evolution restores sight because recreating these conditions essentially swaps his Sharingan from his body to Hagoromo's body.

As to blindness, the Mangekyo and the Eternal Mangekyo, I think Obito clearly proves that a normal Mangekyo is not supposed to lose it's light. Mangekyo losing their light is a weakness, not an intended feature. An EMS is not subject to this because whatever flaw causes blindness is fixed when two sufficiently similar (But different) chakras merge.

As to why this even fixes it, i would say that the donor's chakra stays in the eye, and is reinforced with the acceptor's chakra. This mixture is strong enough to resist wear. On the other hand, Obito's zetsu body was already strong enough to resist wear on it's own.