Dark is very subjective as are most terms used to describe music in general. As Gareth mentioned, are we talking about vibe? Sound as in the opposite of a bright production? Atmosphere/Atmospheric?
If someone said to me that a production sounded dark, I'm unsure I would think that was a good thing as I think of it more in terms of sound - as in the production is too dark or too bright. I can think of several albums that I think have a dark production for what would be considered norms - one of them the latest Dark Empire disk "Humanity Dethroned", for some reason I think it has a "dark" vibe/sound that while I like the music, I'm not fond of the production - the drums are too upfront, the guitars at times a bit muffled/muddy, overall a very unbalanced mix.
Just to clarify - I'm not thinking of it in terms of "dark" heavy metal vs. heavy metal as a nomenclature i.e. sub-genre.
As far as "dark" vibe goes, I'd go all the way back to, in my mind, the first true heavy metal album - "Black Sabbath" by Black Sabbath - it has the darkest, evilest vibe going - the opening to that album sets such an ominous vibe - one of being dropped into a void where there is no return.
What is this that stands before me?
Figure in black which points at me
Turn around quick, and start to run
Find out I'm the chosen one
Oh nooo!
With the "Diabolus in Musica" (Diminished 5th) in the main riff playing along - pure evil
Tony Iommi and Geezer Butler simply write such awesome evil riffs and they set the bar high with the very first song on their very first album.
