Perhaps the acronym "OOF" wouldn't be very fitting considering its current connotation, but something like Out-Of-Focus tags would be very powerful. As an example, some detailed backgrounds have certain elements that many people would blacklist, creating a false positive. For an example of this example, consider those who hate zoophilia. I'm sure that 'feral rating:e' is in the blacklist of quite a few users here. Sometimes that feral is just a squirrel perched on a tree while the actual content that the user wants to see doesn't have any feral-on-feral or feral-on-anthro action. This is just one use case I can think of.

This way, if someone searches 'rating:e -feral', they can still see those pictures with hot anthro-on-anthro action which happen to have a feral animal or a few sprinkled in for decoration or scene-setting. I dunno. I don't think this would ever be implemented; it seems confusing and would only add more work for taggers, since it would be more things to type/click though to differentiate when certain elements are not the focus of the piece. Plus, there would be a lot of ambiguity and opinion on whether an element would be a part of the focus or not. However, it's certainly interesting to discuss what could be, if only a perfect AI could seemingly fetch us unlimited yiff and tag everything perfectly.

I think it's best to work based on actual usecases and, if possible, from existing tags. So, beginning from the case you present, blacklisting feral rating:e: it looks to me like the 'most correct' solution is to instead blacklist bestiality, feral_on_feral and feral -anthro -semi_anthro -taur -human -humanoid rating:e (that is, never show bestiality or feral_on_feral, and never show ferals in explicit-rated posts unless there are non-ferals present). This set of rules effectively blacklists solo feral as well, since a second 'character body schema' tag is required. That should mean, given proper tagging, no sexually explicit feral in focus.
(It wouldn't mean no sexually explicit feral out of focus, but your OOF suggestion wouldn't achieve that either AFAICS)

Some of the *focus tags are also relevant to this discussion, since (feral|anthro|human)_focus are basically the inverse of the 'OOF tags' you are suggesting.