Learn English – What are “faded eyes” like

There's no "real" answer to your question—in this case, it's just some art in writing.

It is not a common phrase in spoken English, and there's no particular phrase it brings to mind. Note that in the phrase, the b alliterates (begins both words)—beetle-black. The alliteration is a poetic way of describing something, and alliteration is often seen in literary English.

But I don't think it's a very good phrase. For one, beetles are not particularly black. They are all sorts of colours. Stan mentions that beetles are glossy/shiny, which is true, but when reading that passage, I didn't think of it (I didn't think of it until Stan mentioned it). You might just as well think of beetles as "covered in dust and dirt all the time."

So in summary, no, there is nothing whatsoever you are missing. The phrase would mean everything it means in English, translated to any other language. There is no "background" you're missing. And do not underestimate the power of alliteration, it makes it sound dramatic even if there's nothing to it!

For both of your numbered examples, I would expect that "blue" is the implied base color. For the sky, blue is the obvious choice; without a further specifier such as "pale grey", blue pretty much has to be assumed. For eyes, a wide variety of colors may be possible, but blue and brown are the two primary choices, and of the two, "pale brown" would not be written as such (it would be "tan", or "tawny", or "hazel"); and any other base eye color would be unusual enough to be specified: "pale green", "pale bluish-gray", etc.