In my modest opinion, dark ambient started with the "Heresy" album by Lustmord and the rest of his production from there. Also Mick Harris Lull project and all the scandinavian monikers like Svartsinn, Kammarheit, Deathwalk, several Nordvargr albums and many more, passing to an enormous worldwide network artists. But I forgot someones for sure.
As sounds, dark ambient is based to obscure atmospheres with dark drones as portant base (bass sound rumbles generated to low frequency sounds, amplified by lush reverb and slow delay and also to audio impulses like sinister noises, breathing voices, disturbant field recordings and many more.
In this sense, I think the sampler is more useful then any intricated enveloped modular synth patches.
Regular ambient I presume it goes to a more "new age" direction.
Not starting a flame war but this is just my personal opinion.
Please, apologize me for my bad english and because I was been verbose.
refuse-resist