dark stuff. i'd say go find anything with a tritone in it. or listen to some alfred wagner. he was a master with dark classical music.
Rothko Chapel by Morton Feldman... Can't see how you'd play it in guitar though... especially acoustic...
There's tonnes of classical music that envokes every different emotion you can possibly think of. Off the top of my head i'd say allan petterson's symphonies can be quite dark and the basso profundo voice in Glass' Koyaanisqatsi gave me goosebumps the first time i heard it.
Does it necessarily have to be classical guitar music or classical music in general? I know some good dark-sounding symphonies and such.