Requested feature overview description.
If a user has SEARCHTERM in their blacklist and performs a search for SEARCHTERM, posts should default to showing as if the user had selected Toggle Blacklist and toggled visibility for SEARCHTERM manually.

Why would it be useful?
Despite not wanting to see SEARCHTERM on a regular basis, users may want to occasionally surface that content, as accepted by the existing ability to Toggle Blacklist. The presence of SEARCHTERM in the search query already serves as affirmation that the user wants to see those results, and showing them in the current results by default would streamline the user experience.

What part(s) of the site page(s) are affected?
This would affect the Posts pages shown in response to a search query.

Blacklists can be a lot more complicated than just having single search terms on separate lines, either by having multiple lines which in some ways work together, or by having individual lines with extra terms or negated terms or other such stuff, and I'm not sure quite how that would work with all of this. Also, how would this work for implied terms? Say I don't usually want to look at penises, and have penis blacklisted. But one day I decide "you know, I really want to look up some art with equine_penis - equine_penis implies penis but as described, wouldn't override the blacklist term automatically.

Dealing with either of these things would be rather difficult and annoying. But not dealing with them would result in inconsistency and annoyance.

I like the idea of this in theory, and it's something I've thought of before, but I'm not sure how it could be done in practice in a way that doesn't cause a whole bunch of issues, and I think it's one of those cases where the benefits wouldn't necessarily outweigh the costs.

The existing Toggle Blacklist functionality works on a per-tag basis, when not used to toggle its entirety. I suggest not deviating from that; that is, considering only lines containing individual tags when evaluating which lines in the blacklist to suppress for the current search.

As for tag implications, a search suppressing a blacklisted SEARCHTERM should also suppress the blacklist for any tag implied by SEARCHTERM. In your example, suppressing equine_penis should also suppress blacklisting of animal_penis, animal_genitalia, and penis. This would not lead to un-sought-for content in the results because the original query already excludes other members of those sets not containing equine_penis.