Well, as Revan says in the Foundry, they were annointed by the Sith Emperor, so I would say that they are Sith. Just as Exar Kun was annointed by Marka Ragnos to be Dark Lord of the Sith.
Now, I have a question. (This is related to the actual Sith-y-ness of Malak.)
While it is known that Revan and Malak were pawns of the Emporer, the question rises: Are Revan and Malak true Sith or just Dark Jedi? While they were under the Emporer's influence, they didn't seem to have complete training in Sith ways, and merely used Dark Side powers. So, is Revan and Malak truly Sith?