Originally posted by Emperor Revan
Like Fishy said, Form 7 was around. He could've (probably) learned Juyo but not Vaapad but that would be a useless form for him anyway since he's already dark side. Look at how Revan holds his lightsaber on the ship before Malak attacks, he also waits for the three Jedi to come to him. I haven't seen that before, maybe he created his own style.
Probably especially after he became the Dark Lord again. With all the knowledge he already had plus Tulak Hord his holocron... Well lets just say that using a conventional style that already existed would be a waste of potential. Revan isn't one to let knowledge go to waste.