Brad Mehldau's Radiohead covers are dark. There are solo & versions with his trio.
http://www.bradmehldau.com/
I also have about 20 soundboards of solo or trio, a few pro-shot videos & an ARTE TV documentary.
& yeah, I know of Diamanda Galas. She's okay, although it really doesn't take a hell of lot of talent to pretend to be possessed....let's be honest. She probably took Mike Garson's piano solo on Bowie's "Aladdin Sane" as her launching pad or at least her stuff certainly sounds that way...it may be dark, but it's in no way virtuosic. I saw her way back when before many of you were even in elementary school on David Sanborn's short-lived Night Music TV show. Another episode, as is well-known now, featured Sonic Youth greatness.