[throwamon]

When dealing with text, couldn't the software replace the area with some text block of Lorem ipsum-style gibberish before blurring it? It could even try to be a little clever and mimic the format of the original text. This way you get the desired effect with absolute secrecy, with the added bonus that you bait people who try to unblur it into wasting their time. :)

[lucb1e]

Certainly the text not being under the blur would be the best way to hide what was under the blur. The trick is of course in actually making it do that in an automated fashion.