So you decide to post a scan of a cheque (check) or a credit card or perhaps even a picture of a travel bug (for the geocachers in the crowd) and you blur out the important number(s). Well with scripting and storage the way it is these days that doesn't really cut it.
Picture someone creating a "dictionary" of pictures with sequential blurred numbers to check against. This would be very much akin to a standard dictionary attack where all the words are passed through the same one way hash and compared against your one way hash. (Create all the possible iterations of the image, blur and compare against your image).
Get the whole story here > http://dheera.net/projects/blur.php.
In the future, black out the number(s) (make sure you don't have multiple layers).