That was quite a thread resurrection!
Dark + Piano pieces, made me think of the one album I have that matches both descrips...Keith Jarrett's 'Dark Intervals'....I spin that disc on sometimes before nodding out at night...Jarrett is up there with Corea and Hancock as Jazz's most proficient and interesting improv key-men.
I tend to gravitate towards minor key music myself...I love the Melancholic side of Steven Wilson and Porcupine Tree, and things like dat...I don't care much for too much Black Metal, but I have a pretty large soft spot for Doom Metal...Candlemass, and the other top names in that genre are about as Dark and Minor Key as your gunna get...Diggin' the new release right now...But, as a noob, I'd definitely suggest King of the Grey Islands, Nightfall or Tales of Creation as an introduction.
I checked out Opening, from Dark Intervals, just before, and I particularly enjoyed the second half. The low end of the piano is so woefully underused that it makes it that much better to hear it again. I'll definitely look into this more.
A visual representation of my music listening since the time of the OP:
http://lastgraph3.aeracode.org/static/graphs/graph_220847.pdf (10mb).
Some of my relevant 'dark music' discoveries since the time of the OP (more than three years), sorted by style:
Prog metal
Opeth: Almost the entirety of their discography.
Riverside: The album Second Life Syndrome.
Pain of Salvation: The album One Hour by the Concrete Lake, and songs like King of Loss, A Trace of Blood.
Symphony X: Not exactly dark, but their music often emphasises what I love most about minor keys. The albums The Divine Wings of Tragedy, Twilight in Olympus, and V: The New Mythology Suite. Example: Egypt.
Green Carnation: Light of Day, Day of Darkness.
Prog rock
Riverside: The album Out of Myself, and the songs I Turned You Down, After, and Before.
Opeth: The albums Damnation and Heritage.
Porcupine Tree: Songs like A Smart Kid, Baby Dream in Cellophane, Bonnie the Cat, I Drive the Hearse, Anesthetize, .3, Heartattack in a Layby.
Steven Wilson: Raider II halfway point onwards, Belle De Jour.
Blackfield: The song Glow.
Neoclassical / Darkwave
Dead Can Dance: The album Within the Realm of a Dying Sun, mostly the songs Anywhere Out of the World, Xavier, Summoning of the Muse.
Elend: The album Winds Devouring Men.
Amber Asylum: The songs Volcano Suite and Riviera.
Middle-Eastern / similar
Arcana: The album Le Serpent Rouge.
Karl Sanders: The album Saurian Exorcisms, and the song Whence No Traveler Returns.
Loreena McKennitt: The songs Marco Polo, Prologue, Marrakesh Night Market, The Mystic's Dream.
Classical / Soundtrack
Ennio Morricone: The Thing soundtrack (as recommended in this thread).
Liszt: Totentanz, primarily this performance
Tchaikovsky: No.6 Mvt.4 primarily this performance
Rachmaninov: Prelude in C#m
Howard Shore: The Lord of the Rings soundtrack, especially the prologue/title music.
Hans Zimmer: Most of the Gladiator soundtrack, especially Am I Not Merciful?, Earth, Progency+The Wheat. Also One Simple Idea from Inception.
Wojciech Kilar: Soundtrack for Bram Stoker's Dracula.
John Powell: Not dark, but minor key fun. To the Roof, from The Bourne Supremacy's soundtrack.
Chopin: From The Pianist's soundtrack, Nocturne In C Sharp Minor (1830); parts of Nocturne In E Minor, Op.72, No.1; Ballade No.1 In G Minor, Op.23.
Javier Navarrete: Deep Forest, from the soundtrack to Pan's Labyrinth.
Gothic Metal
Stream of Passion: The album Embrace the Storm.
The Gathering: The song Travel.
Female-vocals
Sissel: The songs Lær Meg Å Kjenne and Sofdu unga astin min.
Emiliana Torrini: Gollum's Song, in Howard Shore's soundtrack for LOTR.
Anneke van Giersbergen: Valley of the Queens, in the Ayreon album Into the Electric Castle.
Loreena McKennitt: The songs Skellig and Dante's Prayer.
Stream of Passion: The song Nostalgia.
Guitar-driven dark ambient / neo-folk
Matt Uelmen: Diablo II soundtrack, primarily the songs Rogue, Tristram, Wilderness.
Tenhi: The album Kauan, example: Etäisyyksien Taa.
Vàli: The album Forlatt is very melancholic, for example, Doedens Evige Kall, Et Ensomt Minne, Dypt Inne I Skogen.
Spanish Guitar
Ewan Dobson: The song Blood and Ice. Also of interest is Acoustimetallus Plectrus and Legend of the Brown Goat (these are not Spanish sounding).
Rodrigo y Gabriela: Not exactly dark once again, but heavily emphasises the minor and dissonance. The albums 11:11 and Rodrigo y Gabriela. Example: Atman.
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It has become apparent to me over time that the term 'dark' is used to describe some very different things in music. I use it in three senses:
1. Describing music which simply 'sounds good' owing to the minor scale (or more exotic scales), dissonance, steps of a single semitone up or down especially, and sometimes dramatic feel, e.g. Rodrigo y Gabriela - Atman.
A lot of prog metal, film soundtracks, middle-eastern influenced music, and Spanish influenced guitar is like this.
2. Describing a very melancholic, depressing mood. E.g. Porcupine Tree - I Drive the Hearse
3. Describing an oppressive, sometimes otherworldy mood :smiley_emoticons_mr. E.g. The Thing soundtrack
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The last kind is the hardest to find, at least, in my experience.
My last.fm:
http://www.last.fm/user/zxc2323