I think we call it "dark" because the term was coined when the English language was used in a more poetic sense - at least it seems like that to 21st century-me. "Dark" = "it has not been made visible to us".

I've just been reading Narnia stories to my son and lots of the language seems dated and initially confusing but very descriptive and more poetic. Even though that was just in the mid-20th century.