Sorry I havent listened to the track, but as far as I know.. Dark dnb is all about using the 'darker' sounding scales & chords, as opposed to the happy sounding major key.
In other words, some intervals between notes sound happy, and some sound evil.
Not to get too bogged down on music theory, but an example would be playing the notes C and E together. This is a major 3rd interval and sounds very nice and happy. Then play the notes C and D# together. This is a minor 3rd interval and sounds 'dark'.
Although much to the tradition musicians dismay, many DNB artists have ditched the scales (or perhaps just noobs who have no music knowledge) and just write simple melodies that sound 'good'. Hell.. theyve even got a word for playing the wrong notes of a scale.. its called 'Chromatics' and is often used to create a feeling of tension in a song.
Dont take my word for this though.. I am trying to remember the theory from years ago. You should do a search on the grid about this.