The Black Thing is the like Sauron, Darth Vader, and Voldemort all rolled into one, and it's coming to a planet near you. But what, exactly, is it? Calvin has the same question:
"But what is it?" Calvin demanded. "We know that it's evil, but what is it?"
"Yyouu hhave ssaidd itt!" Mrs. Which's voice rang out. "Itt iss Eevill. Itt iss thee Ppowers of Ddarrkknesss!" (5.108-109)
Thanks, Mrs. Which, that really clears things up. So we know the Black Thing is Eevill, but beyond that, zilch. We don't know where it came from, we don't know what it wants (other than to take over the universe, naturally), and we're not even quite sure what its relationship to IT is.
Why is A Wrinkle in Time so vague about the Black Thing? Well, part of it may have to do with the book's theme of there being more to the universe than what is seen: just as Mrs. Whatsit & Co. are beyond the power of Meg's puny human brain to comprehend, so too might be the Black Thing. Another reason might be a rule well-known to horror-movie fans: a danger is scarier the less of it you see. Give away the monster in its entirety, and you say goodbye to the chances of Cloverfield: The Sequel.
On top of all this, having the Black Thing simply be pure, unadulterated evil gets rid of any pesky moral quandaries in fighting it. As with the Nazis in the Indiana Jones movies, you can root for the good guy with full confidence that the villains he's destroying deserve to be destroyed. Presenting the Black Thing as Evil, full stop, makes the good guys look all the better by comparison, and makes their quest an inarguably righteous one.
But why then does the novel want to make the side of right so absolutely unquestionable? We'll leave that as an exercise for the reader.