Is it possible to make the whole screen to flash with some bright light through a script?

I want something like that blinding sun light we see in many Bethesda games when you leave an underground place for the first time, where the player becomes partially blinded.

It might be possible with an 'image space modifier'? I dunno much about how to set them up though.

it certainly is.
basically, choose if it animates (fades etc) or not, if it animates, you enter a duration in the box. the slider then represents your timeline.
for an easy example, say it's all normal at start and all black after 1 second. so you just slide to the end, set it all dark and when applied, it will linear morph between the states and after that switch view back to normal.
if you want more "keys" (states to morph to), put the slider to your desired position and change anything there, whenever anything at any time position differs from the previous key, it will be a new key (you can click through them with the arrows).
if you want the screen to stay altered (instead of going back to normal) after the timespan is over, you'll have to apply a second, unanimated modifier (or expand your animation time to very long with a second key with the changed settings at the end - beware though, changing time after adding keys will scale the whole timeline.

for the image settings, i won't even begin, but i got all i know about it from the geck-wiki, so best look it up there. they're a pain to handle, but you can really do stuff of far out psychedelity with these things )

Yeap, imagespacemodifiers are what you want! I use a blinding flash effect in my Meteor spell. You'll probably want to create one that jumps to pure white in 0.25 seconds or less and adds some blur, then slowly fade back to normal over the course of several seconds. Heck, there's a lot of stock imods already, you may find one that does what you want. You can preview them by loading up a cell, then highlighting the imods. SAVE OFTEN, I get more crashes while messing with imods than with any other part of the CK.

Edit: Here's an example of the sort of the "bright flash" blinding effect I'm talking about: http://www.youtube.com/watch?v=8DzYcsF23og

In mine I'm fading to yellow and orange, if you're doing a sunlight blinding effect you'll probably just want to tweak the brightness and bloom settings.

Thanks guys. Already know how to do it. Helped a lot.