The darkness will feel the same way to her as the void does on the eastern part of town, that same malice and drawn without her having to be on the other side of the wall to feel it. She can tell that the darkness isn't the lack of something, but rather, an encroaching something the same way the rest of the void is an encroaching something.

Since Merlin is a part of this thread and is keeping track of the void's spread, I'll add that by Day 3 he will definitely be able to tell that it's eating away closer towards town. It's not uniform along the entire length of the void's edge, and the largest amount of loss is only by a couple of inches, but there's movement and that's probably worrying, maybe.