There is this regarding Eol...
The special title "
Dark Elf" given to 'Eöl of Nan Elmoth seems to refer to his personal aversion to the light of the Sun rather than his heritage:
"
But now the trees of Nan Elmoth were the tallest and darkest in all Beleriand, and there the sun never came; and there Eöl dwelt, who was named the Dark Elf. [...] There he lived in deep shadow, loving the night and the twilight under the stars."
And...
Tolkien said that in his earliest conception, the Dark Elves were "imagined as wandering about, and often ill-disposed towards the 'Light-Elves'". Later he considered an additional use of the term: "sometimes applied to Elves captured by
Morgoth and enslaved and then released to do mischief among the Elves." He thought that this latter idea should be taken up for Eöl.
From here...
https://tolkiengateway.net/wiki/Moriquendi
Still as the page you linked says it was also a general term for any Elf who had not seen the Light of the Trees.
Sometimes, however, Tolkien used it more to mean just the Avari, the Elves who had refused the invitation to Valinor.
But even among the Avari were antagonistic towards the Eldar, but that didn't mean they fought on Morgoths side, and some of the Avari were friendly towards the Eldar. The bulk of the populations of Mirkwood and Lorien are a mixture of Nanorin Eldar and Avari.
And yes sometimes Dark Elf was used as a special title for Eol (who sometimes was an Avari and at other times a Sinda)
But even though he comes the closest to the modern fantasy version of Dark Eles/Drow, he wasn't really any more evil than Feanor, he was just a bit darker and creepier (okay a little bit more evil) and was not aligned with Morgoth.
As for the enslaved Elves Tolkien later drifted away from them. He decided later that Elves had the ability to have their souls voluntarily escape from the body (so effectively dying) than endure that. Or in the very least he decided that there were a lot fewer Elves that werer enslaved by Morgoth than he originally envisioned. At one point he had the concept that, almost all Elves were enslaved by Morgoth after The Battle of Unnumbered Tears, with Gondolin and Thingol's people being the only two large factions that escaped that fate.
In the same vain he also decided that it's impossible to rape an Elf, since again, their soul would just eject itself from the body. That's also one of the reasons why Tolkien later wanted to get away from the idea that Orcs are corrupted Elves.
Of course that still leaves Maeglin who did work with Morgoth on some level and surrendered Gondolin's secret location...
And the part where he said that the feral souls of Avari Elves were the ghosts and phantoms called upon by necromancers. And since Sauron is the greatest of necromancers...
