This is more a question of literature than history. Blindness is the most common deformity among seers and oracles, and it's a metaphor that works on multiple levels. The first is that wisdom has a price -- nothing comes free, especially not the gift of prophecy. The second is that only those who shut out the immediate can see what lies beyond.

Let's take Sophocles' Theban plays, Antigone, Oedipus the King and Oedipus at Colonus for an example.

In these, Oedipus is praised for his "clear sight," but in reality he's blind to what's going on around him. Tiresias, who is physically blind but a prophet, forecasts what will happen to Oedipus. Only when the aging Oedipus blinds himself does he gain limited prophetic vision.