I'm curious to learn why Obito didn't turn blind when he used the Mangekyō Sharingan?
It was because he was injected with Hashirama's cells.
Background: Madara, defeated by Hashirama Senju, took a big bite from Hashirama before he died. However, he implanted Izanami to one of his mangekyou sharingan to alter reality, such that he was back alive. He used Hashirama's cells from the chunk he bit off to cultivated more cells and treated himself with those cells to elongate his lifespan.
Madara found Obito after he was crushed under a boulder and treated him with cells he cultivated from Hashirama Senju. Apparently, those cells not only elongate one's lifespan and heal very fast, but also suppress the side effects of excessive mangekyou sharingan use.
This may be one of the several reasons:
Hashirama Senju was most famous for his healing jutsus. Any wound he received healed seemingly effortlessly.
Hashirama Senju was the reincarnation of Asura.
Hashirama Senju could use the wood release, the only release that can create life.