There are whole categories of niche stuff that most people don't seek out, and that many people generally won't like. This is one of the strengths of image boards--being able to find stuff to your taste, even if your taste is obscure (within the general parameters of the board--probably not much Picasso here, tentacles or no). So, maybe many people don't particularly like tentacle porn, but I don't think that's a good criteria for blacklisting it by default. Many people don't like urushihara_satoshi, too.

If we started allowing yaoi, I'd probably put that in as a default, and maybe "pregnant rating:e" is a reasonable one (which would, incidentally, cover the original image), but in general I don't think expanding this list is the answer.

My current plan is to think up a way to make adding to your own blacklist more convenient and obvious, since it's buried in the account options right now.