John Milton's "On His Blindness" is an autobiographical poem in which Milton contemplates the loss of his sight. Given that he was not born blind, Milton knows what it is like to have sight and misses his ability to see. His blindness has made him feel useless.
(On a side note, most speakers in poems are normally defined separate from that of the poet. In this case, one can assume that Milton is the speaker given the poem's autobiographical nature. It is for this reason that Milton is referred to as the speaker of the poem in this answer.)
Many different words signal (and support) how Milton felt regarding his loss of sight. "Dark," "death," and "useless" all denote Milton's feelings about his blindness. It is certainly not a happy place for him. Instead, he feels lost given he is unable to see the things around him. He is also fearful. Milton feels forsaken by God, illustrated by the idea of "light deny'd" (7).
Milton finds himself desiring to follow God more "now" than before, yet he recognizes the fact that it may not really matter. Milton knows that God will reign because he has "thousands at his bidding" (12) and he (Milton) does not matter in the grand scheme of things. Therefore, Milton feels some lack of importance in the whole of things.