It seems as if this expression is used in a lot of different ways and with different meanings. Who can explain to me what this really means? I need to translate it to German, but any english explanation would be greatly appreciated.
To me, it is somewhat of a humerous exaggeration. I don't think it's a common phrase, but the meaning I take from it is that the person in question finally realizes something that is painfully obvious, that is, something that he or she should have known all along and something that the average person would know as well.
I agree with JediMaster. The image used is of lightning, so that the truth suddenly rushes on you in one great flash of illumination, in which what was previously very dark and confused becomes suddenly terribly obvious... The humour lies in the fact that a big drama is being made of something you ought to have known perfectly well all along.
It's often used to describe speech, eg 'You mean I'm stupid!' said Louisa, with a blinding flash of the obvious....