Answered by Shaykh Faraz Rabbani
One of my friends is legally blind. Why is it disliked to pray behind him?
There is an important principle, explained by Imam Abu Zaid al-Dabusi, that, A general ruling need not apply to all particular cases.’
It is generally somewhat disliked ( makruh tanzihan) to choose a blind person as imam because they may well not be as able as others to avoid filth ( najasa), may veer a little from the qibla, and may miss washing their limbs completely in wudu. [Shaykh Zada, Majma` al-Anhur, 1.107, based on Mulla Khusro’s Durar]
However, if the blind person is more knowledgeable, more righteous, or generally more suitable, then it is not only not disliked for them to be imam but rather superior for them to be chosen to lead. [Ibn Nujaym, al-Ashbah wa’l Nadha’ir, 3.344; Ibn Abidin, Radd al-Muhtar, 1.560]
This is why the Messenger of Allah (Allah bless him and give him peace) chose Ibn Umm Maktum (Allah be pleased with him), a blind Companion, as acting governor of Madina (and imam for prayer) when he (peace and blessings be upon him) went on expedition to Tabuk. [Shaykh Zada, Majma` al-Anhur, 1.107, quoting al-Burhan]
So, for example, one of my teachers, Shaykh Hassaan al-Hindi of Damascus, is both blind and imam of a mosque. This is common.