The key is how many G's you're pulling. An untrained, average Joe might well black out visually at 5 G, especially if he's not doing a proper G-straining manuever. He probably wouldn't lose consciousness for another couple of G's or so though. So in that case, he could probably control the plane (although a real plane isn't going to go where you think it's going if you fly it with your eyes closed, while a virtual plane can be flown somewhat "accurately" while blind).
However, presumably we're simulating "trained, battle-hardened" fighter pilots here. A fairly good guide would be: onset of tunnel-vision, about 5-6 G, tunnel-to-blackout, about 7G, loss of consciousness/control, 8G (9G tolerance is for guys wearing G-suits, in 30-degree ejection seats)
You could also add a "severity penalty" for maintaining a blacked-out (cold) state for more than, say, 10 seconds. In most cases, pilots who've actually GLOC'ed take several MINUTES to regain sufficient awareness to actually fly the plane effectively. I don't think we should model it as minutes, but being a "passenger" with controls frozen at neutral for, oh, 20 seconds or so would be a pretty good "object lesson."
While it may be POSSIBLE to fly while blacked out, doing it at the G-levels modeled in the game would be a RARE exception, rather than an acceptable simulation.
--jedi