In Empire’s crawl, ‘lord’ isn’t capitalized, making clear that it is a title, not a name. I don’t think Vader has ever been referred to as ‘the evil darth Vader’, where the ‘darth’ isn’t capitalized. That would make it clear that it was just a title. But then again, Jedi is always capitalized even when saying ‘the noble Jedi Luke Skywalker’, so a capitalized Darth may be the naming convention used for Sith lords.
But why use the ‘lord’ at all if all Darths are Sith lords? Wouldn’t using ‘Darth’ suffice? It can’t be to distinguish Sith masters from Sith apprentices, since Dooku and Vader are both the apprentices of Sidious and yet they are both referred to as Sith Lords.