Board Thread:Questions and Answers/@comment-6780495-20140402113726/@comment-24480380-20140405145355

216.27.165.146 wrote: Ragna's not 'corrupted' by Saya, his Black Beast within him just went out of control. He retained his sense of self and his mind while it was happening and he repeatedly told Jin and Noel to run. Actually, what happened was that Izanami forced Ragna's Azure Grimore to activate and go out of control, so that there would be seithr in the world for her to use.

I suppose you could say he was "Corrupted" by Saya in some sense, but whether or not he's actually under her control remains to be seen.