The Moriquendi (the Elves of Darkness, singular Moriquende) are, in essence, the Elves that did not join the Great Journey over the sea and behold the light of the Two Trees in Valinor. They lingered on the shores of Middle-earth or, indeed, never passed the Blue Mountains on the east of Beleriand or the Misty Mountains further to the east. Those Moriquendi who originally started on the journey but for some reason did not finish it are known as the Úmanyar, "Those not of Aman".
It is a term for Avari, Sindar, and Nandor, but in practice is only applied to the Avari. An exception to the hiarachy of Elvish peoples is Thingol, who, though he did not complete the journey, had earlier been to Valinor. Thingol was therefore considered Úmanya but not Moriquende.
- "Those Elves the Calaquendi call the Úmanyar, since they came never to the land of Aman and the Blessed Realm; but the Úmanyar and the Avari alike they call the Moriquendi, Elves of the Darkness, for they never beheld the Light that was before the Sun and Moon." -- The Silmarillion, chapter 3, "Of the Coming of the Elves and the Captivity of Melkor"
The term "Dark Elf" in Tolkien's writing seems to hold an additional special (not explained) meaning, as it is given as a special title of Eöl of Nan Elmoth. It is also used as an insult by the Sons of Fëanor to Thingol (which is incorrect, since Thingol alone of the Sindar was reckoned under the Calaquendi).