The theme is universal - we all want to make things sound better than they are. That's why when someone asks you "Does this dress make me look fat?" you always say no, even if it does. We want the world to sound better because if we really focused on how bad the world is then no one would want to live in it. But not all writing does make things sound better than they actually are. Some writing is really depressing and makes things sound as bad or worse than they are.
The example is a metaphor. It means that the hills are dark. Hills usually aren’t actually black. It doesn’t use like or as and it doesn’t use a human quality on a non-human thing.
