There's a lot of ways to refer to folks of all sorts of complexions with many areas of typical use. Some use foods, nature, industrial, poetic. Depending on how dark the skin is depends on what words you might use. Such as someone who is "as dark as the night sky" or "A living, breathing void of beauty". Those of a decently dark complexion might be compared to a "gorgeous mahogany" (more so red mahogany), a skin as "deep and rich as the earth" (though in some cases being compared to earth can be a bit insensitive, think on the themes of the story and if it fits and speaks on the theme as well). As you get a bit lighter you can have a bit more flexibility as well. "His rich walnut skin shone with a warm underhue" can also be such a thing. Writing in the appearance of underhues as light shines through the skin can be telling as well. "A beauty of such a dark complexion, that when the sun met her it gave an unearthly blue glow to her features." These are some of the ideas I would have on the subject, but even I could be fault to tropes that may be less pleasant, so hopefully we can get some more interaction with this post from a more knowledgeable audience :)