The title of the John Milton poem now often referred to as "On His Blindness" was originally called "Sonnet 19" by Milton himself. When first published in a 1673 collection of poems (plainly titled Poems), it was listed as "Sonnet 19." This changed roughly ninety years later when the composition was published again in a 1761 scholarly edition of Milton's poetry entitled The Poetical Works of John Milton, with Notes of Various Authors. This edition was edited by the Anglican bishop Thomas Newton, who had produced an annotated version of Milton's magnum opus, Paradise Lost, in the past, as well as a massive scholarly work on biblical prophecy.
Newton was the one who assigned the title "On His Blindness" to "Sonnet 19" in his edition of Milton's poems, and that title has stuck to the work ever since. Some readers prefer to use the poem's first line, "When I consider how my light is spent," as its title, but the shorter title invented by Newton remains the more popular option. The Newton title has likely lingered because it is brief and to the point regarding the subject matter of the poem, even if it is historically inauthentic to Milton himself.