dark prog rock

There are even more dark prog nowadays:

spaced out

interference sardines

thinking plague

indiscipline

sleepytime gorilla museum

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MUSEO ROSENBACH's "Zarathustra" from 1973 is quite dark (the MP3 in the Archives being the lightest piece on the album), which one would expect given the subject (Friedrich Nietzsche's "Thus Spake Zarathustra"). It's worth trying, carl!, as it fits both your stated criteria. Check out the reviews of the album in the Archives.

Lists by The Owl and Greenback are quite significant: I would add ART ZOYD, NEBELNEST, HOYRY KONE, FAUST

diddy wrote: King Crimson Morte Macabre Anekdoten Museo Rosenbach Van der Graaf Generator Peter Hammill Univers Zero Simon Steensland (partly)   some of the "dark" bands I recall at once...

Certainly support Diddy's list here - especially the earlier Anekdoten Vermod and Nucleus. However, while Univers Zero is dark musically you will be shifting into the very distinct and different branch of prog - RIO.

You asked about records from the 70's, try Spooky Tooth/Pierre Henry's Ceremony - probably ST's only prog album. Also check an earlier album, the self-titled first album by White Noise - the last two tracks (which made up side 2 of the original vinyl release) are dark, especially compared with the relative daftness of (the original ) side one. And some recordings released by Blue Oyster Cult, have their moments of dark progginess.