I think that is what we call "figura de linguagem". It happens mainly in written (text, books etc). It is a way to say something and given a deeper meaning to the sentence. Many authors use it a lot. In this sentence if the author had used the word "escuro" probably the sentence would not have caused the same impact as using the word "negro". But, as Aline said, both have the same meaning. When the author use the word "negro" give us the sensation that there was really really dark. He couldn't see absolutely nothing.

In this case, negro and escuro are synonyms, when you mean “dark”. I think escuro is more commom in spoken portuguese in Brasil. And I think that this sentence is in portuguese of Portugal.