I vaguely recall that "Dark Lord" was a title generally only given to Voldemort by those sympathetic to him. Which is not to say that Draco's a full-blown DE at this point, of course, but rather that he might be more comfortable with that title than Ginny Weasley would be.

Draco uses the term 'Dark Lord' at the end of GoF.
And yes, pauraque, I'm afraid I'm going to read H/D subtext into it. ;)