When going to Negative Y level in a world with the usual pre Caves and Cliffs Part II size, meaning from 0 and deeper, a slight black shadow would appear around the player's screen.

But now with the new Negative Y level where the world still generates, this shouldn't happen, unless you're in the actual void.

In the images below you can see my character in Positive Y level, without the shadow in the screen. Then in Negative Y level without the shadow in the screen. But by going deeper one block (From Y=-10 to Y=-11) the "Shadow void" suddenly appears in the corners and sides of the screen