The first eight lines of the poem "On His Blindness" have an abab rhyme scheme, meaning that, divided into two four-line sections, the first and fourth lines of each section rhyme, as do the second and third. Thus, in the middle of each four-line section, there is a rhyming couplet.
The first couplet rhymes the words wide and hide, and the second rhyming couplet rhymes the words chide and denied. These rhyming couplets, as part of a regular abab rhyme scheme, lend to the poem a pleasing, lilting rhythm, which in turn perhaps reflects the positive and uplifting message of the poem. The message of the poem is that it is "Kingly" to persevere and serve God as best as one can.
There are also throughout the poem several examples of alliteration. In the second line, for example, there is alliteration in the phrase "this dark world and wide." In this instance, the alliteration of "world" and "wide" helps to emphasize the scale of the speaker's blindness. His inability to see is especially daunting precisely because there is such a big, "wide" world that he is blind to. The difficulty of his blindness is emphasized in correlation to the scale of what he cannot see.
There is also an example of alliteration in the line "Doth God exact day-labour, light denied?" In this line, the speaker is melancholy and disconsolate and begins to doubt God's benevolence. He seems to question how fair it is of God to expect him to carry on working while at the same time denying him the "light" of the world. The alliteration in this example perhaps helps to convey the speaker's disconsolate tone. The soft l sound, repeated twice in quick succession, lends itself to a soft and melancholic tone.