There are various explanations as to why Yitzchak became blind in his old age. The best known reason is that when he was bound on the altar by his father, the angels cried. Their tears went into his eyes, and eventually caused blindness.
There is also the possibility that Yitzchak was blinded by Eisav’s idol worship. As part of his worship, there was a great deal of smoke, that filled Yitzchak’s tent, from the incense burned. This caused loss of vision.
The Torah warns against taking bribes. Yitzchak was bribed by Eisav’s fake piety. The bribe is capable of “blinding the eyes of the righteous.”
The final explanation comes from the Talmud in Megilla 28a. There is a warning not to look at the image of an evil person. If one repeatedly looks evil in the face, it could cause loss of vision. Not only are we to distance ourselves from evil, we should also not even look directly at evil.
If our great Patriarch, Yitzchak, could be so negatively affected by his association with evil, we certainly need to take great care to separate and insulate ourselves.