Playing something that sounds dark has more to do with your tone and how you play. You can make the major scale sound dark. One chord arpeggio the sounds kinda dark is the m/maj7 arp. Used alot in harmonic minor and melodic minor tunes. Say C is your starting note (root), the sequence is C, Eb, G, B. Played harmonically, as in a chord, this is the simplest, most freightening sounding chord there is. It's actually used in alot of horror movie scores. The diminished arps are quite dark also. Add this to the m/maj 7 arp by playing this...


Cm/maj7 Bdim7
|-------3-------3-|-------4-------4--|
|-0-1-0---0-1-0---|-0-1-0---0-1-0----|
|-----------------|------------------|
|---------1-------|---------0--------|
|-3---------------|-2----------------|
|-----------------|------------------|


Basically if you want to write darker stuff your going to have to just play until you find something that sounds cool to you. Playing melodies using alot of minor intervals will lean to a darker sound, also tri-tones are good stepping stones from phrase to phrase.