I’d appreciate if you could stop using /s/Poaldev for personal suggestions. Moved.
Now to answer your questions:
1 You get a prompt telling you that you can't post in subs you've blocked.
That’s already the case when you try to submit a new post and manually type the blocked sub’s names (because they won’t show up in the drop down list). I don’t see the point in typing their names. Only subs you can post in will ever be listed. No more, no less.
2 It shows up in your posts because it is your post.
All posts (including yours) submitted in subs you decide to block afterwards won’t show anywhere because the filter is about sub blocks, not post blocks.
(post is archived)