[Image-SIG] GIMP-like Sobel filter?

[tdc]

Hi,
I'm trying to make some automated image processing using PIL. My goal is 
to locate largest blocks of JPEG encoding artifacts in source image. 
I've made some prototyping in GIMP, but I'm unable to mimics these in 
PIL. Examples are at http://www.webprostor.cz/media/tdc/ I'm looking for 
some (fast) way to get results displayed in Step #3. Localization of 
largest "blob" of black is the next step, but first steps first :)

Any ideas?

_tdc_