What is the role of blindness in Milton's Samson Agonistes?

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,....

What stylistic elements in Milton's "Samson Agonistes" relate to blindness?

A complete stylistic analysis of portions of Samson Agonistes is, in this confined format, of course impossible, but I can take you through an analysis of ten lines related to blindness.

So fond are mortal men
Fall'n into wrath divine,
As thir own ruin on themselves to invite,
Insensate left, or to sense reprobate, [ 1685 ]
And with blindness internal struck.

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,

The first thing you'll notice about theses ten lines is the lexical point that only three lines start with multisyllabic words: insensate, Semichor, despised. Of these, one is a proper noun, and the name of the speaker, while the other two are an adjective and a verb, insensate and despised respectively.  Another lexical point readily noticed is the use of words related to fire or light: illuminated, extinguished, fierie ashes, flame, and, the metaphoric representation of spiritual light, divine.

A prominent syntactical point is that sentence word order doesn't always follow the SVO/C/A (Subject Verb Object/Complement/Adverbial) options for standard sentence order. Looking at the essential form of the sample quotation sentence illustrates this:

So fond are mortal men fall'n into wrath divine, ... And with blindness internal struck.   

The archaic meaning of the adjective "fond" is foolish or silly: So foolish are mortal men .... The word order we have in this sentence is VSC (Verb Subject Complement). We have an adverb (so) modifying an adjective (fond) preceding the linking Verb (are) followed by the Noun Phrase (mortal men), forming the sentence Subject, followed by the compound Complement (fallen into wrath divine and with blindness internal struck). The second part of the Complement (and with blindness internal struck) inverts the word order in several regards.

The compound be verbs, are fallen and are struck, are split, first by the imposition of the Subject (mortal men), then by the imposition of the prepositional with phrase (with blindness internal). The with phrase also has word order inversion. The adjective internal, in Standard English, should precede the noun blindness. The second part of the compound Complement would therefore be: struck with internal blindness.

So fond are mortal men
Fall'n into wrath divine,
As thir own ruin on themselves to invite,
Insensate left, or to sense reprobate, [ 1685 ]
And with blindness internal struck.

A point of phonological interest in the passage immediately above is the switch from emphasis on open vowels to emphasis on sibilant consonants and closed vowels. "As thir own ruin on themselves to invite," is the transition line combining the open characteristics of the first two lines with the upcoming sibilant and closed characteristics of the next two lines. /o/ and /a/ in the first two lines, as in {fond} and {fall} have open back sounds that are warm and relaxing. The nine instances of /s/ in the transition and two end lines add tension through the sound of forced air escaping. The closed /e/ phonemes add to the tension produced by the sibilance. These phonological choices support the meaning of the text and heighten the emotional response to the text.