The repeat count determines how many times to process the image or region. The Randomization Percentage determines if any given pixel is to be blurred. The Seed is used for random number generation, offering you the opportunity to get truly random blurring from one use to the next. For each pixel, a random number is generated based on the seed value and compared against the randomization percentage. If the random value is less than the randomization percentage, then the pixel is blurred.
Smaller percentages mean fewer pixels are blurred.
If the random numbers generated are truly random, then you should get the percentage of pixels blurred you specify with the randomization percentage. These two blur filters are very similar in nature but are better suited to different types of original images. Both filters have a very simple dialog that allows the user to set the Horizontal and Vertical blur radius, either equally are individually. In the upcoming 1.2 release these filters also allow you to specify the units to use for these lengths, ranging from pixels to picas to inches and beyond. It is expected that, over time, most of the rest of the blur filters will also permit this sort of unit selection.