According to developer interviews in Final Fantasy XV Official Works, Ignis only lost his eyesight rather than his life due to his strong will to protect Noctis, a person the kings of yore could not afford to lose.

How did Ignis get blind?

Ignis doesn't go down without a fight and reveals that he stole the ring of the Lucii back in Altissia and puts it on, burning his eyes and going blind.

Can Ignis drive blind?

After said scene, most of the game is spent on other things, but we do learn over some cut scenes that (more spoilers!) after ten years has gone by, Ignis has maintained his blindness, and is also kicking ass doing it.