"On His Blindness" begins with a complaint, which Milton's speaker then amends and reframes. The speaker's "light is spent"—that is, he is blind—and blind man might reflect bitterly on his fate and ask why he has been singled out to be deprived of one of life's blessings. Milton changes this obvious point to a question about how he is to serve God adequately when he cannot see.
The form of the Italian sonnet is perfectly adapted for Milton's question and answer here. The turn comes in the middle of the eighth line and unfolds through the sestet. God gave the poet the gifts he has. He does not need them returned to him. However, the speaker can still serve God's purpose, first by bearing what he has to bear without complaining and second by standing ready to serve God when God requires his service.
The central idea of the poem, therefore, is that the speaker—and, by extension, his audience—should serve God willingly with whatever gifts are given to him, and that service might mean waiting patiently. One could argue that the poem's rebuke to the speaker's initial self-importance trivializes the poetic gift Milton displays in his absolute control of the sonnet form.