Why does the poet call the talent useless in "On His Blindness"?

That reference is found in these lines:

When I consider how my light is spentEre half my days in this dark world and wide,And that one talent which is death to hideLodg'd with me useless . . .

At this point in his career, Milton had gone blind. The word "talent" here is a Biblical allusion to the Parable of the Talents, in which one man is punished by God for hiding his talents away and not using them for the Lord's work.
Milton sees his blindness as burying the gift of poetry within his soul. He feels that God has gifted him with this ability of creating poetry to use for His work, and Milton longs to use his talents for God's purposes, saying that his "soul [is] more bent/ To serve therewith [his] maker." In this moment in his blindness, Milton (presumably speaking as himself in this poem) considers his God-given talents "useless" if he does not have the vision he feels he needs to complete the work, yet he presses forward so that he can present a "true account" of his efforts to God when his time comes.