Oh no, I don't read too many webcomics. I probably wouldn't become a constant reader.
I'm just saying that from an artist point of view, and someone who reads many hard copy comics, that every frame should have at least a little bit of black in it to create an easy base for the eye to perceive depth within it. The only time there shouldn't be black, is if it is done for a dramatic effect of sorts.
It is hard for your eyes and brain to judge how much something should stand out, or how to visualize dimensions of a piece of art without a solid base. Whereas everything is gray, that makes it VERY hard to do, and causes changes in the way things are seen between each frame.
Another thing here. Look at the real world. Almost EVERYWHERE you look around, whether anything is black or not, your mind will see shadows and those will seem black, because your mind is making a base. You can't force that into a piece of art though, so when creating the art, you have to place it there for the viewer.
The point is. Whether your constant readers notice the change or not is indifferent. The best way to draw people in is to have something that makes them go. OH. It will also make people more appreciative of your art, as well as make it look at least a hundred percent better.