Including blacklisttags:removein a search would have a similar effect as copying your entire blacklist into the search bar with a minus sign - behind each tag. In other words, posts that include blacklisted tags or have the property of being blacklisted will not only be hidden, but will not be retrieved by your search query in the first place.

Including blacklisttags:add in a search would have a similar effect as copying your entire blacklist into the search bar with a tilde ~ behind each tag. In other words, any post that includes a blacklisted tag or has the property of being blacklisted will be included in your search. This is useless in a normally as it has the same effect as an ordinary search, but if your search only includes blacklisttags:add then the effect will be the same as searching for any post that contains a tag you have blacklisted. As a further example, blacklisttags:add plus order:score will return all posts covered by your blacklist in order of score.

You may have, on occasion, performed a search that returned mostly blacklisted results. The page you are greeted with is populated by only a few posts, and browsing all of the non-blacklisted ones requires either going through page after page to find the few posts that were not blacklisted, or taking you blacklist and manually copying sections of it into the search bar. Having the blacklisttags:remove metatag would remove this issue and allow you to receive a full page of non-blacklisted results with minimal effort.

As for the circumstances under which the blacklisttags:add part would be useful, I'm not sure, but there are probably a few circumstances under which you would want to view only posts you would otherwise have blacklisted. At the very least, it would be the negatory complement of the blacklisttags:remove metatag, in the same way order:score_asc exists as a counterpart to order:score.

The search bar (and the help section would include information on the new meta tag).

1. Using this metatag might either circumvent or cause issues with the 40 tag search limit. A potential solution would be to have each individual tag that is pulled from the blacklist count as it's own tag search, contributing to the limit, even though the user wouldn't see all of them. Another is listed at the end of issue 2.

2. Blacklists often include - as part of their tag syntax, such as in young -rating:s. Searching blacklisttags:remove essentially adds a minus in front of every tag in your blacklist and includes them in your search, and searches such as -young --rating:s completely remove anything tagged young regardless of if it is rated safe or not, and blacklisttags:add, which adds a tilde to every item in your blacklist and searches them, creates issues I can't even begin to imagine.
I don't know of any solutions to this I'm confident in because I can't see the backend of this site, but if I had to guess, I would say that it might be possible to treat my statement earlier

Including blacklisttags:add/blacklisttags:remove in a search would have the same effect as copying your entire blacklist into the search bar with a tilde ~ / minus - behind each tag.

as a metaphor, and instead of literally pulling tags from your blacklist and invisibly including them in an ordinary search, it would request the servers to only display posts with the property of being blacklisted or not blacklisted in the same way that votedup:anything requests the servers to only display posts with the property of being upvoted or not upvoted by you. I know that blacklisted posts must be distinguished by a unique property at some point along the line because their thumbnails are automatically replaced with the "blacklisted" image. In fact, this might solve issue 1 as well.