He reacts before he is consciously aware of the danger, folding wings and dropping a few feet like a stone while pulling his cloak over his face and neck, avoiding all but the fringes of the blast....
Or anything else you can think of.
Evasion...so he's evading the blast to some degree. It is tough to imagine how though, since the rogue never actually leavs his 5' spot while evading it. Then again, combat is imperfectly modeled by turn-based actions anyway, so turning a blind eye to what would seem a logical problem with him evading it is usually a good idea.
DM2