Unknown to many fantasy readers, there
are Dark Elves in
J.R.R. Tolkien's
Arda, though they aren't named in his
most famous work. The Dark Elves are the
Moriquendi. As noted in that node, these are elves that never lived in
Valinor, and they are considered "dark" (prefix "
Mor-") for never having seen the
Two Trees.
The Moriquendi are a perfect example of Tolkien's ability to provide grays in his work - blurry edges that haze the world and give it a deeper sense of realism. Countless conflicts arose between the Dark Elves and the "
Elves of Light." They were usually "political" - feuds, hatreds and friendships for the most part driven by honor, oaths, or curses.
The Dark Elves are not necessarily "evil," but are... well... darker. Black hair and grey eyes, living in the forests and paying reverence to the
stars, they built the first kingdoms of Middle-Earth, prospering until the
actions of the Noldor brought war to their lands.
Unlike Dark Elves in subsequent fantasy works, the Moriquendi of the
Silmarillion are only dark
by name and are not, by default, the enemies of Good. There are slight
exceptions to this, but in Tolkien, only the creations of
Morgoth are not to be sympathized with. All Elves are children of
Eru.
Note: Terminology such as "dark elf" or "grey," "green," or "wood" elf is usually a Tolkien-ish representation of the Common Speech, and interpretation may vary according to context.
For instance: by definition, Elrond is a "Dark Elf," Legolas also, but Galadriel is not. But Elrond is a descendent of Valinorian elves, and he's the son-in-law of Galadriel (who dwelt in Valinor herself). In this case, Legolas would be the only true Moriquendi.
A hobbit like Meriadoc Brandybuck would not know the difference (except by the color of their hair), and elves like Legolas, Elrond or Cirdan would not be called "dark" unless extremely corrupt.
Other Dark Elves of note (that have been noded): Amroth, Nimrodel, Elu Thingol, Luthien Tinuviel