I also have experienced this extremely odd bright almost blue WHITE flash of light. Actually, I have had it happen three times over a period of about ten years. Like you, I have sought answers with no certain success but have a theory that may work in some of mine. There is a phenomenon called thermal lightning flashes that is a little like bigfoot sightings. Science just declares it to be heat lightning from thunderstorms that are too far away to hear the thunder. That is their pat answer and the end of any interest in actually studying the reports of it.
What I've seen is not a flash from a distant thunderstorm. It is just very different in its duration and the color is slightly different. Then too there were no thunderstorms anywhere within a hundred miles on at least two of the occasions and once it was in the late fall and cool. The flash is, to me, a little like a carbon arc or welding flash from arc welders. Intensely bright lasts a little longer than a lightning flash and as you pointed out leaves the impression that it didn't make shadows.
On one occasion it actually happened several times over about a five minute period. It actually was bright enough to wake me up. I thought at first that someone was in the bedroom with a bright flashlight when I first woke up. Then it was gone. It lasted long enough though to wake me and still be happening when I opened my eyes. I turned on the lamp beside the bed. There was nothing there. I turned to turn the lamp off thinking that maybe it was a car light that somehow shined into the bedroom window and I had just misinterpreted it. It happened again before I could turn the light out. The flash seemed to originate from outside but the light almost seemed to come from all around me at the same time somehow and not cause any shadows. Every corner of the room seemed to be bathed in the same white, almost blinding light.
That got me up and out of the bed. I grabbed a flashlight and went outside wondering if I had something going on out there. As I went to the front door it did it again and was the same effect in the living room through the windows faced in a different direction than the bedroom windows. I went out into my front yard looking around. I remember looking up at the night sky and seeing bright stars from horizon to horizon. They were the special bright that you have on a cool night where there is no heat rising anywhere to distort their pinpoint sharpness. Then it happened again. The entire sky flashed and everything was lit up just as it had been in the house. It was so bright that it totally washed out any light from street lights. After that, the show was over I guess. I sat on my porch for 15 minutes waiting but it didn't repeat.
First of this massively bright light was made with not even a whisper of sound. The second odd thing about it to me was that even though I describe it as almost blindingly bright, it left none of the after images that that sort of bright flash normally leaves. A flash from a camera leaves an image etched onto your retina that takes a little while to fade. This didn't do that.
I wonder if this isn't something like the Saint Elmo's Fire or lightning balls except on a larger scale and a shorter duration. If the flash was indeed from "all directions" that sort of uniform brightness would leave not after image on your retina since it illuminated EVERYTHING with the same brightness and would make no shadows. I believe that it is a little as if you were bathed in light to the point of actually being immersed in it somehow.
Maybe it is some form of atmospheric charge that is then triggered and the light is coming from each individual molecule all at the same time. the light from each is tiny but the accumulative effect would be massive. My only other thought is as Deb mentioned, some sort of retinal occurrence that sort of whited out the entire retina surface at the same time. I have experienced this sort of thing a couple of times in a reverse sense where it just went black.
Your retina is a little like the old tube TVs that took a second to warm up before the picture forms and came into focus. It isn't instant on or instant off. In my blackout experiences, it was caused by pain. BLINDING pain is a real thing. You are there and conscious but the world just fades to black and you are blind. This is actually caused by your brain shutting down and then "rebooting" over and over as you are fighting and near to losing consciousness but note going out and staying out. The experience is a little like sleep paralysis and scary as hell.
If your brain can black out your percieved vision I see no reason that it couldn't white it out for an instant. ??? in no case did I sense, see or perceive any paranormal activity. Maybe it is just a poorly understood and rare natural sort of occurrence. LOL, all of this is purely conjecture. and of no certain factual information and is just my effort to unscrew an inscrutable happening.