It's used as part of a set phrase which acts as an adjective. That being said, it's easy to imagine an adverb answering the question "how", as in, "How dark is the sky?"

It's used as part of a set phrase which acts as an adjective. That being said, it's easy to imagine an adverb answering the question "how", as in

Q: "How blue is the sky?"
A: "It's dark blue." <- possibly an adverb, with the meaning that it's mostly black and a little bit blue

or

A: "It's slightly blue" <- definitely an adverb