I was wondering when someone was going to mention VDGG... that's about as dark as it gets. The mid-era Generator (Godbluff, Still Life, World Record) are just amazing.
Crimson goes without saying... any of the pre-Belew/Levin albums and definitely less-than-happy, but not angry. Tense, perhaps. Flower Kings are way too saccharine even for me. Good call on Genesis... The Knife, The Musical Box, The Return Of The Giant Hogweed... Cinema Show might be an interesting contrast to Damnation. Basically, any of the stuff before Banks and Collins strapped the leash onto Hackett... for three albums he was the band, and he's still The Man.
You may also wish to try some Fish-era Marillion, dark but self-absorbed. Or something I've been jamming lately, Versus X... like a not-so-sunny Flower Kings meets ELP, and they at least came up with ONE of the all-time riffs (about the 2:00 mark of Curtain Call).
My first vibe off Damnation (don't laugh) was like the one I had when I first played "Sap" by AiC; tonally similar, and like Damnation was to Deliverance, a very unexpected change of course from the previous Facelift.