Just a little theory id like to express 
I've always found it strange that neither Obito, Izuna, or Shisui experienced the eye damage Itachi, Sasuke, and Madara did. And I think the reason for that is because the latter three could use Amaterasu, which causes direct damage to the user's eye (everytime its used the eye bleeds after all). With Amaterasu's usage, the user's eye degrades more and more. Kamui seems to be able to damage non-Uchiha's eyesight after overuse with Kakashi, while Madara seemingly brought the eye back to full condition after he took it.
So anyway, Amaterasu is what causes the eye damage and blindness ultimately. If a Mangekyo user lacks it, they don't go blind.
prove me wrong with proper manga scans which suggest otherwise
I've always found it strange that neither Obito, Izuna, or Shisui experienced the eye damage Itachi, Sasuke, and Madara did. And I think the reason for that is because the latter three could use Amaterasu, which causes direct damage to the user's eye (everytime its used the eye bleeds after all). With Amaterasu's usage, the user's eye degrades more and more. Kamui seems to be able to damage non-Uchiha's eyesight after overuse with Kakashi, while Madara seemingly brought the eye back to full condition after he took it.
So anyway, Amaterasu is what causes the eye damage and blindness ultimately. If a Mangekyo user lacks it, they don't go blind.
prove me wrong with proper manga scans which suggest otherwise
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