You misunderstand a couple of things, I think...
First is, I'm a subscriber just as you are. Cayars is also a subscriber and both of us have nothing to do with development. We both tried explaining what we have now, comparing this to the desired functionality. Elan is a developer and his "official" response from another thread is the one I paraphrased above.
Second, we agree. Black List exclusions added to the current White List exclusions makes much more sense and provides a greater amount of control of our media. It doesn't seem that the Plex Team (developers) wants to give us that much control over our media or our hardware. This control is either broken, not well thought out or outright missing that could provide us with the tools you, Cayars and I are discussing here.
Cayars (Carlo) and I are agreeing with you completely. There has been discussion in other threads here for this very function, but I was singled out that it was a one-off and others wouldn't use/need it. (For a function I actually have no immediate need for, which I find the ultimate in irony...) :)
All we can do with the current system is provide you with the workarounds we have found. We have only two tools to exclude specific media from sharing with specific users:
1) Enter in the "Included" flags in the improperly labeled "exclusions" fields for the user, making sure we touch all 2800+ files affected to ensure they include these flags and that the 200 or so we don't want shared don't contain those flags. Or
2) Make a whole new library with the "unshared" media in it and make sure that the user we want to exclude it from doesn't have that library shared with them.
Neither are ideal, and both are much harder to maintain a large collection on. These are the only choices we have available to us, at this time. Now you have to figure out which you want to use, until we get the actual tools to do the job correctly. I offered you the option to add your vote/voice to this by looking for a request for black listing on the Feature Requests forum and clicking on the green "Like" button, as this is the current method of voting.
Here are some of the existing threads already asking for something like this:
https://forums.plex.tv/topic/130476-feature-request-restrict-content-based-on-multiple-criteria/?hl=restrictions
https://forums.plex.tv/topic/129565-feature-request-option-to-exclude-labels-for-home-users/?hl=restrictions#entry777406
I'm sure there are others. These were a couple of the first finds, and neither have a lot of votes ATM.