Theophylact of Ohrid

But the blind man himself also did not have perfect faith. Therefore the Lord does not give him sight all at once, but gradually, as to a man whose faith is not complete, for healings are given in proportion to faith.

Commentary on Mark ↗

Bede

Seeing indeed the shapes of bodies amongst the shadows, but unable to distinguish the outlines of the limbs, from the continued darkness of his sight; just as trees standing thick together are wont to appear to men who see them from afar, or by the dim light of the night, so that it cannot easily be known whether they be trees or men.

Catena Aurea by Aquinas ↗