totally... all of these things too. great production advice.try using long notes, low pass filtering and play your instruments in a lower octave, make the song sound slower than its tempo. maybe throw some distortion, long reverbs and dub delays in the mix too.
Actually most of that dark stuff in hip-hop/rap is made using progressions that move minor-diminished-minor-diminished, e.g.
Am-G[SUP]#[/SUP]dim-Am-Bdim-E[SUP]7[/SUP]-Am
[mp3]http://www.bandcoach.org/fp/audio/dark.mp3[/mp3]