Except they use magic. In fact, you need to promote a magic-using class to be able to change into one. Mage first, knight second.
If they were a class that promotes from cavalier or knight, or w/e, it would make more sense just to tag on the ability to use anima magic, but they're not. They're promoted from magic classes, including dark mage. Why should they not use dark magic?
A dark mage changing to a dark knight, and not being able to use dark magic makes absolutely no sense and almost ruins the dynamic of the class. --- http://tinyurl.com/a8lyhvl