Just to say that i've never read salvatore and my interpretation of dark-elves and duergar were that they were "fallen" members of their species.
In the case of the d-elves they have such long lives and elaborate social structures that some began to follow corrupted forms of this set up. They are dark in as much as they followed the dark path just as wood elves aren't made of wood.
The duergar were corrupted by another more magical means (for example the dark crystal in the base of mortorn) and their values became skewed (ie the love of gold in dwarves is not offset by other things in the duergar make up).