ok so, i think my ideal implimentation would be this: your instance acts as a post office.

your client addresses things, under the hood, to 'everywhere you havent blacklisted, and also where the instance has blacklisted' unless you specify it only to go to specific addressees (followers, mutuals, @)

this also would make it easyish to impliment local-timeline-only posts, or cluster similar-minded instances into 'herds'