The first one is correct and common. The second one means you're not afraid of all that the abstract concept of darkness means. Hope this helps, Lisa! :)

First one would be correct if you are talking about a fear. When we use the noun ‘darkness’ it sounds very obscure and like you may be using a metaphor for other things you are scared of, rather than darkness itself.

They're both correct, but I'd say "I'm not afraid of the dark" is more natural and common.

They are both grammatically correct. But I also agree that the first one is more natural.

The difference is that nobody has ever actually said the second.