Masashi Kishimoto's Kakashi's backstory arc, sacrificing himself to protect his masked rival from a huge boulder and donating his one remaining eye as a final gift. For a long time, Obito was assumed to have died that day, but he was secretly saved by none other than Madara Uchiha. The legendary ninja and his Zetsu assistant tended to the youngster's wounds and replaced his missing flesh by using the First Hokage's renowned DNA. Initially intending to return to Konoha, Obito's plans changed after witnessing the death of his beloved Rin - a tragedy that activated his Mangekyō Sharingan. From there, Obito posed as the hapless Tobi and later as Madara himself in a conspiracy to resurrect the real Madara.
Only a select few from the Uchiha clan ever unlock the Mangekyō Sharingan in Naruto. The powerful eye ability is painful to achieve, requiring the death of a loved one as a trigger, but once awakened grants the techniques far superior to that of the basic Sharingan. Utilizing these techniques does not come without risk, however, as overuse of the Mangekyō Sharingan can lead to total blindness over time. Itachi's vision is already on the brink while battling his younger brother, and Sasuke has Itachi's eyes implanted to cure his own failing sight, creating the Eternal Mangekyō Sharingan. Even Kakashi revealed he was struggling to see through his borrowed Sharingan towards the end of Naruto.
Throughout his post-resurrection life, Obito uses the Mangekyō Sharingan's abilities liberally, regularly deploying Kamui to enter an alternate dimension. Despite this, there is never any sign of Obito's sight deteriorating or the same blindness that afflicted Sasuke and Itachi. This could be seen as a Naruto plot hole, but that's not necessarily the case due to the cells of Hashirama that were first used by Madara to save Obito.
The inner workings of the First Hokage's cells are never fully explained in Naruto. In truth, they mostly serve as a convenient plot device that cures people (such as Madara and Obito), grant powers (Yamato's wood release or Danzo's creepy arm) and performs other such miracles as needed. But because of the wondrous properties Hashirama's DNA can bestow upon a person, it's plausible that the cells didn't just heal Obito's boulder injuries, but also prevented (or at least delayed) the blindness that usually comes with using the Mangekyō Sharingan. Kishimoto never outright confirms that Obito didn't go blind for this reason, but if the Senju juice can heal fatal wounds and sustain Obito so well he never needs to eat or drink, it can surely stave off the curse of Uchiha blindness.
In the overall scheme of Naruto, not properly explaining Obito's sight isn't hugely problematic. Having said that, addressing the plot point, even in a throwaway line, would've expanded on the lore behind the Uchiha clan and the Mangekyō Sharingan in particular, and make the double-edged sword of the eye technique more prominent, rather than just exploring the flaw in the context of Sasuke and Itachi.
