In "An Essay on Man," why did Pope say blindness to the people is kindly green?

In “An Essay On Man,” Alexander Pope writes to explain the ways of God to man. The poem is somewhat an homage to Paradise Lost by John Milton, who also wrote that he was attempting to explain the ways of God to man; therefore, the poem can be seen in the same light, as an attempted theodicy—a way to explain evil and why God permits it in the world. Any time that someone is writing to “explain the ways of God to man,” they are partaking in the genre of theodicy. The part of the poem you have quoted comes from the third section, where Pope is writing about fate and the future.
The section starts with Pope explaining that humankind cannot see the future or understand their fate because men wouldn’t be able to suffer their mortal lives knowing that there is a heaven and a future in eternity. The idea of humankind's limitations being a blessing is not a new idea—it stems from the concept of Felix Culpa (fortunate fall), in that humankind can be saved and live in Christ because of the fall of Adam and Eve and the limitations of being mortal.
The part you’ve quoted comes from the second half of the first stanza of section three. The line reads,

Oh blindness to the future! kindly giv'n,
That each may fill the circle mark'd by Heav'n:
Who sees with equal eye, as God of all,
A hero perish, or a sparrow fall,
Atoms or systems into ruin hurl'd,
And now a bubble burst, and now a world.

The point is that blindness—not real physical blindness, but spiritual blindness—is given to people so that we might be content in our lives on Earth. If we could see the future or see everything that God sees, we would not live the simple life that leads to salvation—namely, a life of suffering and humility for the sake of Christian transformation. The blindness, therefore, is kindly given to humankind, that we might fulfill our purpose set out by God—the “circle mark’d by Heav’n”—to grow into the image and likeness of Christ. The blindness, our inability to see beyond our lives, protects us from the pride that leads to the downfall of men and angels.
Pope’s mode of writing is deeply influenced by Christian symbolism and theology, and his poem is deeply entrenched with the imagery and beliefs of the Christian church of his time. It's essential to understand how deeply those beliefs played a part in the logical construction of his ideas. Unless one sees knowledge as a means to damnation—something that the enlightenment would later argue against—it would be nearly impossible to understand what Pope means by the fact that blindness is kindly given.