And yes Alrik Aventurian usually cant tell elves apart, so night elf agents are a thing, especially in the north.
Oh boy you'd be in for a surprise. True, the Dark Elves are not really common. I mean, elves in general aren't really common to begin with. But if there is a region where people know the Shakagra and can differentiate them from the Firnelves, then it's in the north. Because, you know, they bot live there, and one will kill you and the other won't and it's pretty good to know how to differentiate them. Even if it's just "this is the figure of one story and this one here sounds like the figure of another, darker story."
Considering them as a playable culture (I believe that's what the question hinted at)... yeah, no, they're not meant for that. They're even worse then Orcs in this regard.
With "Blades of the Night" you most likely could built one, yes (the Adventure was basically an excuse to fleshed out the culture of them by the author) but they're not really supposed to be playable characters.
They're bad. And I mean, really bad. They're basically physically disgusted by any non-shakagra, especially elves. They live in deep undersea cities, in pitch black darkness, except for some eery bioluminescent fungi and things. They don't really like fire and the sun harms them. At least the old generations. Because they're not "one people", there are several different "tribes" and actual different dark elves, since Pardona kept on creating and "optimizing" them. The culture is on the verge of a civil war between different believe systems (Pardona as a kind of prophet and one go compared to a more direct worship of the nameless one). They hate the other gods and think everyone else is inferior. And with hate not I just don't mean "don't like them" I mean hate hate, as in they will actively go out of their way to harm or desecrate ay references to other gods. Not to mention that they're slavers, and to the nice fun kind like Al'Anfa. If they catch you, the BEST that can happen to you is that you work till you die (considering the lack of light and food and the hard labor, that'll take a month max, usually) and that your body is then eaten by your fellow slaves. If you're exceedingly lucky or skilled, you might even be fed some eyeless deep see fish (raw). The worst would be that your body is turned to some monstrous diamond creature (or different parts of you into different monsters - while you're still alive of course) and your should to be sacrificed to the nameless one.
Even IF you could come up with a story that would make sense for your char to not be in their cities anymore, and you survive their persecution of you (they're not really the kind of people who'd shrug their shoulders to something like that), you'd still be in the middle of nowhere in the endless ice - which brings down even an Shakagra quite fast. Now if we say you'd make it into "civilized" territory (that means still something like the far north east of Canada or Greenland), you're still dead meat mostly. The Firnelves (and all other ones, too, because you're basically the embodiment of everything that's bad - badoc - to them) will kill you without a second guess, the Fjarninger (think Conan style barbarians) will definitively kill you, basically all settlers from the middle realm will kill you on sight as well, because nothing good ever came from a dark elves. You might have more luck with the Nivesen, because they're not great on killing people, but even there I wouldn't try my luck to hard. Oh and the Thorwalians will kill you, too, and they actually know about you because of the saga of Philleason.
If you'd somehow manage to come further to the south, you actually might have a chance to be not liked on sight (if you don't run into other elves, orks or any blessed ones - because you're still a being more or less directly made by the power of the nameless one), as people really don't know elves from one another. However, here you will have problems with basic living. Food, temperature, probably even the air you breath are either completely unknown to you and in many cases even harmful to you.
So yeah... until this changes (which MIGHT happen, as mentioned Pardona tries to improve her works all the time and the Shakagra are on the verge of civil war), they're not really playable.