I assume from my experience that you call the users blacklist with the onload event but i would suggest you call the users blacklist every time a new set of images is loaded (different pages of images from the 'next page' button). For most users it is not noticeable but i have an addon from chrome that allows for infinite scrolling where it will place all pages on the same webpage (only loads next page once you reach bottom as to not call too much from server) but only the first page set of images is effected by the blacklist because it is (probably) only called when the web page is loaded.
This would be useful to just make the site all around more flexible and user friendly and would allow a native infinite scroll feature if you were to ever implement it.
The only part of the site that would be effected are the search results and would most likely not have any performance impact and would probably speed up the process of removing the blacklisted images.
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