How does darkness work?

There's a really lacking description of how the darkness works and what it protects against.
It's just that 1 in 10 hits from mobs or bosses simply pass through it, but when I check to see if it's gone, it's halfway through.

For example, the boss of the second interlude, I caught almost all of his attacks and it didn't take away much darkness, but after catching one slash I survived, and the other died.

So, here's the question: do any defenses affect the darkness? Is it even necessary to build any type of defense? What does the darkness affect and what doesn't?

P.S. my resistance to all elements was no more than 30%