Auras are not atypical for migraine sufferers. In fact, migraines with auras are referred to as “classic” migraines. For me, an aura usually starts out as a tiny shimmering spot in the center of my vision. It looks a bit like the after-image you see when someone takes a flash photo of you. Instead of fading like the after-image from a flash would, the spot slowly grows. As it gets bigger, I can see that it has details: it is a colorful shimmering crescent wrapped around a white circle. Gradually, over the course of 20 minutes or so, it grows until the white center fills my entire field of vision. I’m temporarily blind. And then, over the next few minutes it slowly fades away until my vision is back to normal.
via ironicsans.com

I'm really glad to have stumbled across this post, as I have always wondered, and worried about the "blank spot" in my vision that I get before a migraine.
While that doesn't make the whole experience any more pleasant, at least I know that the visions are not early warning signs of a stroke, or anything else so sinister. I'm just normal, as annoying as that usually is.