To compliment the existing per-user Blacklist
I'd suggest implementing as an extension, or more specifically a pre-check on the blacklist.
When the site goes to evaluate the images in a given imageset for blacklisting, it would first check each image against the whitelist. Any image that matches the whitelist would then be excluded from any further Blacklist processing.
My suggested use for this is for that *one* (or several? I'm sure other people will come up with their own interesting uses) artist who's work you certainly don't mind, but who manages to hit multiple lines of your blacklist on a regular basis.