Understanding the poetry of Ralph Waldo Emerson is a bit easier if you first learn about the man and his ideas. Ralph Waldo Emerson (1802–1882) was a graduate of Harvard Divinity School and, like his father, became a Unitarian minister. After only a few years, he found that his personal beliefs and values did not mesh with many church teachings, so he eventually left the ministry. He never quite left the pulpit, though, because he was very well known in his day as a popular public speaker, as well as essayist and poet. Emerson also was one of the founding leaders of a new, radical philosophical movement called Transcendentalism.
Transcendentalists believe that each person is a microcosm of the entire universe and that everything is connected and part of the whole. A central idea is that one's direct experience of God is unique to each person, and can't be defined or dictated by religious doctrine. In addition, Transcendentalists argue that science and technology tend to hide the unity of all things, rather than explain anything truly meaningful.
Emerson's "Blight" is a lament about people's increasing reliance on science and scientific method (the blight of the title) at the expense of their own innate knowledge as part of a great universal whole. The poem is a passionate call for humanity to give life its full measure of depth and meaning by rejecting materialism and giving up its reliance on scientific methodology as the only way to find truth.