Well, in a case like that, they see what they see. If they attack the scourge without investigating them, and at least roughly know their strength and numbers, then they are going in blind, and it is well within your right to keep them in the dark.
But there is another thing to consider. It's hard to get it to work, but if it does, I think it can really add to the game. Remember, you are telling the players, not the player characters, what is going on. The player characters see what they see, nothing more. But the player can make an informed decision about what he wants his character to do. A wizard player can stay off the magic until he can unleash a big spell to eliminate most of the scourge with one hit. Yes, that's pretty powerful, but it's also pretty cool, and at least to me, seeing cool stuff happen is a big part of why I play.
If you can get away from the "GM vs. Player" mentality, I think it is a great thing to do. But if the players feel that they have to fight you, instead of their characters fighting the BCV, you are probably better off with keeping them in the dark.