Is there a way to make it so that one cranky viewer that doesn't like a particular pairing/ship/whatever can down-vote something and blacklist an image?
Yes, I know that, but the question is if it gets negative votes, the image automatically gets "Blacklisted" status and is then no longer viewable -- even if it's just one. This makes a personal dislike into a community problem, since one person disagreeing quickly can make all of us unable to view an image.
To fix that you can adjust your comment threshold under the Options tab, which is automatically set to zero. I have mine set to -1000. This keeps stuff from disappearing when people refuse to blacklist.
Well, you can actually also re-view an unviewable image by clicking a little "[no. of unviewable posts] hidden" button in the down-right corner. Though it's not very noticeable.