John Chrysostom

And what follows also indicates, that of themselves it was said, and of their traditions. Thus He added, "They are blind leaders of the blind." Whereas, had He spoken it of the law, He would have said, "It is a blind leader of the blind." But not so did He speak, but, "They are blind leaders of the blind:" freeing it from the blame, and bringing it all round upon them. …

Homily on the Gospel of Matthew 51 ↗

Jerome

Let them alone, they are blind, leaders of the blind. But if a blind man leads a blind man, both will fall into a pit. This is what the Apostle commanded: Avoid a heretical man after the first and second admonition, knowing that he is perverted and condemned by himself (Titus 3, 10). In this sense, the Savior also commanded that the worst teachers should be left to their own discretion, knowing that they are difficult to be drawn to the truth, and that they are blind and lead the blind people into error.

Commentary on Matthew ↗

Thomas Aquinas

There follows let them alone, they are blind. Here he shows that their scandal is to be disregarded because they are harmful to men. And first he teaches that it should be disregarded; second, their presumption; third, the harm. Regarding the first: you say that they are scandalized; let them be, and do not concern yourselves. But should one not be concerned about scandal? Did not the Lord, to avoid scandal, send Peter to the sea to pay the tribute? …

Commentary on Matthew ↗

Rabanus Maurus

They are blind, that is, they want the light of God's commandments; and they are leaders of the blind, inasmuch as they draw others headlong, erring, and leading into error; whence it is added, If the blind lead the blind, they both fall into the ditch.

Catena Aurea by Aquinas ↗