For what it’s worth on the newer versions of Lemmy with the ability to import settings files, you can create and share json files of blocked instances/communities without overwriting other user settings. Not as streamlined as what you’re describing, but it’s an option given current circumstances.
E.g.
blocklist.json
{"blocked_communities":["https://lemmy.site/c/meh","https://lemmy.site/c/mehbutmoremeh"],"blocked_instances":["unpleasantlemmy.site","lemming.mean"]}
I finally got around to testing this and found that it doesn’t overwrite existing blocks, merely adds them to your existing list. I made sure that the import file only contained new blocks and not duplicates to verify. You have to refresh the page to see the changes, and may take a few seconds depending on list length/instance performance, but it works.