What is the black thing that Mrs. Whatsit shows the children on Uriel?

A stopover on Uriel is a welcome opportunity for Mrs. Whatsit and the children to take a much-needed rest before continuing on their journey. Uriel is a beautiful, peaceful planet, a place of safety and security. It has the added advantage that it allows one to get a good look at The Black Thing from a safe distance.
In A Wrinkle in Time, The Black Thing is a massive dark shadow that represents all that is evil in the universe. The Black Thing completely covers a number of planets—Camazotz being one of them—and on each of those planets evil reigns unchecked. Other planets, such as Earth, are mercifully free from the grim shadow of this evil power, but it's by no means certain that things will remain that way.
The implied religious allegory is quite obvious here. The people of planet Earth are presented with a clear choice between good and evil. If they choose the latter, then the planet will—metaphorically, at any rate—become shrouded in darkness. But we haven't reached that point yet, and until we have, there's still time. Uriel or The Black Thing? Good or evil?