"Two Jedi acquired a taste for rebellion after their victories during the Mandalorian Wars. They discovered Pre-Republic artifacts that led them to the Sith tombworld of Korriban. Despite their lack of a connection to the Sith species, they drew upon the knowledge from the Sith tombs and declared themselves the new Dark Lords of the Sith- Darth Revan and Darth Malak.
(Something not directly about the Jedi Civil War.)
It seems the true line of the "Dark Lord of the Sith" died with Exar Kun. Revan and Malak declared themselves Dark Lords of the Sith, as did Traya, Sion and Nihilus. (Though the Dark Council also seems to have a tradition of of all members being called "Dark Lord of the Sith")
This also seems the point where the extensive use of the Darth title started. Most of the old Dark Lords weren't called Darth, IIRC. Exar Kun didn't call himself Darth either.
Maybe the Sith Emperor or the Dark Council started this traditions and Revan picked them up there.