Can you use the darkness to hide?

In many stealth games you can hide, and lay bodies in the shadows / dark places to remain undetected.

Can you do this in Dishonored, or the only thing that counts is cover behind objects?

You can use darkness to hide, actually, but it works only when enemies are far from you. Don't do anything too loud, and you can snipe them out easily. Shoot with a gun and they'll be alerted and run toward the noise.

If they come where you're hidden, you'll be visible. It's explained on the tips shown during the loading screens between levels. Line of sight is still your better option to be invisible, but shadows are useful to perform unseen movements in big areas with not much coverings.

I wanted to provide an in-game screenshot, but couldn't find one. I instead found a link and you can check the 3rd point in the list for confirmation:

Shadows won’t save you. Most stealth games task you with using darkness to cover your movements. But Dishonored puts the emphasis on line of sight — that means you’ll want to keep objects and structures in between yourself and enemies as much as possible, peeking out using the Triangle button. Shadows still help conceal you, but only to distant enemies.

Hint: where there's a shadow, there's usually an object creating that shadow. This object, 90% of the time, is big enough to give you line of sight coverage.