The Black Thing is the embodiment of evil. Evil is often described as "dark" or "black" in literature. The Black Thing has been opposed by great artists, religious figures, inventors, philanthropists and thinkers throughout Earth's history. Basically, the Black Thing is a Devil-like character battled by Christ and other religious figures, because the novel references Biblical versus. Buddha is also mentioned. It was also fought by other important thinkers. Leonardo da Vinci, Shakespeare, Einstein, Bach, and Gandhi are also mentioned.
By describing the Black Thing as having been so important to various historical figures, L’Engle is establishing the common thread of humanity that fights with evil. It is not just religion that is important to battling evil and establishing good, but also art, science, philosophy and good works.
In the book, the Thing is a representation of pure evil embodied by a very dark cloud which completely blocks out light. This Thing that threatens the entire universe has already taken over planet Camazotz and is currently moving in on earth and has covered part of it. In fact, on earth, the Black Thing has been opposed by prominent figures such as Jesus, Rembrandt, Buddha and Bach over the centuries. In Comazotz, everybody thinks and acts simultaneously because their minds are being controlled by IT, a huge live brain that dictates evil and hate. It is in this planet that Meg’s father is trapped because he failed to yield in to mind control by IT. Since the Thing knows only evil, Meg uses the power of love to defeat it.