What most people call "dark ambient" now, didn't really exist when I got into ambient music. When people talked about "dark ambient" 10 years ago or so, they meant ambient music that was darker, deeper, and slower... stuff like Lustmord, Thomas Koner, or Trances/Drones. More like what I'd now refer to as "night ambient," not meant to be spooky, just sedate and subdued. By this definition, stuff like Oophoi or Vidna Obmana or even my own work might be considered "dark ambient."
Then a bunch of black metal crossover artists sort of took the words "dark ambient" and it became sort of industrial-ish, abrasive ambient with overtly "spooky" or aggro elements.
I don't know if I'd say it didn't exist 10 yrs or so ago. Think about early(ier) Tangerine Dream stuff, like Green Desert for one. Fabulous album, but fairly dark from what I remember of it. A fair piece of their earlier stuff could be considered dark ambient, even though a lot was/is sequenced.
I played in a doom metal band, I've been into extreme metal now for 18 yrs or so. Prior to that was Trower (still!), Rush, Deep Purple, Iron Maiden, Judas Priest and the list can go on and on. I still listen to that stuff when I'm in the mood (nostalgia some times, especially for Rush/Trower as they are my two favorite guitarists). Some of the black metal crossover artists as you pointed our are so-so, some good, some not so. I like the dark ambient for the mood it tends to bring across, I just let my mind wander/relax listening to it. Same goes for Doom metal (prefer extreme to stoner - I don't even know why it's lumped in with Doom as it has nothing to do with it). Doom doesn't depress me at all, I love the stuff. Same for dark ambient, moving, emotive, scary, pensive, thought provoking. At least that's what I get out of listening to it. Same with most other forms of music.
In a bad mood? Crank up Meshuggah - Rational Gaze (or pissed off at neighbor ;-) ) as that's a killer song and my guitar instructor commented at how they put a lot of effort into that song with regards to structure and vocal delivery. To me, that says a lot coming from him as he's old school/classic rock type....haha, I pummel him with brutal stuff now and then ;-)