By the time that the
dark jedis were exiled into the
sith territory, they used their dark powers to make themselves kings of the
sith. So they came to be knowed as Lords of the Sith. In the time the Sith Empire grew up apart of the
Republic, a time
a.k.a The Golden Age of the Sith, the greatest Sith Lord alive in a given time was know as Dark Lord of the Sith.
After the Great Hyperspace War, the title maintened his status, but often there was more than one dark lord, and of course, internal war, to see who would be the sole Dark Lord of the Sith. After Darth Bane's law, the title was aplied to all sith acolytes that survived the training time. So Darth Sidious, Darth Maul, and Darth Vader were all Dark Lords of the Sith.