We call it "dark" because we are sloppy.
The same side of the moon always faces the Earth. If the same side of the Earth always faced the moon, then only one hemisphere would be able to see the moon. Since you can see the moon from everywhere on Earth (not at the same time...), we know that the same side of the Earth does not always face the moon.
Unless you're thinking of the "outside" of the Earth. /s
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.