One way to determine the role of blindness in Samson Agonistes is to examine all the instances of blindness in the text and analyze them for thematic value. Since the answer format of eNotes does not permit a full examination,  we can look at a couple together to get started. The initial reference to blindness occurs in "The Argument" that precedes the text. In it, Milton describes the argument predicating (i.e., forming the foundation of) the text starting with an introduction of Samson's condition, situation, and present circumstance.

• Samson is imprisoned and blind.
• As a prisoner, Samson is a  laborer "as in a common work-house."
• Samson has gone apart  from the others to a quiet spot in the fresh air to "bemoan his condition."

Besides describing a realistic situation, this also describes Samson's metaphorical condition and symbolizes a greater spiritual truth reflected in his error. While he previously was free, powerful and favored by God, he is now as much a spiritual prisoner of his error and as spiritually blind as he is physically imprisoned and blind. Therefore the role of blindness is set up as dualistic: blindness describes Samson's physical state while equally revealing his spiritual state.  

Samson made Captive, Blind, and now in the Prison at Gaza, there to labour as in a common work-house, on a Festival day, in the general cessation from labour, comes forth into the open Air, to a place nigh, somewhat retir'd there to sit a while and bemoan his condition. (The Argument)

This role is confirmed by Samson's later lament to his friends who have come to his side. He explains that the blindness he once thought was his worst affliction, he now feels is his least affliction. He says that his shame is such that, if he were to have his sight, it would do him no good for he would be unable to hold his head up or look up at anyone.  He says this is because he has betrayed the "secret gift of God." Thus, his previous spiritual blindness, which led him to wreck the "vessel trusted to [him] from above" (his life and strength), causes greater suffering than the physical blindness that he now counts as least.

Yet that which was the worst now least afflicts me, [ 195 ]
Blindness, for had I sight, confus'd with shame,
How could I once look up, or heave the head,
Who like a foolish Pilot have shipwrack't,
My Vessel trusted to me from above,

The final report of Samson's fate made by Semichor after Samson has wrecked the colosseum confirms this role of blindness as a metaphor and symbol of inner spiritual blindness. Semichor expresses this by describing the antithesis of sightlessness after Samson’s strength returns and he redeems himself. Semichor says that Samson, though physically blind, was once again "illuminated" by virtue that burst "into sudden flame" that "reflourishes" in Samson's last act. Samson was blind spiritually and physically; at the end, his spiritual sight is "illuminated," though his physical blindness remains.

Semichor. But he though blind of sight,
Despis'd and thought extinguish't quite,
With inward eyes illuminated
His fierie vertue rouz'd [ 1690 ]
From under ashes into sudden flame,....