I’m just following up to report that the code in the .htaccess file is working now – I used the code as is (i.e. just the word “blog”). I think my issue was caused by a cache-related delay.
I still see an error in site health but, despite that, the plugin seems to be working, e.g. blog users can be found and followed on Mastodon, and their blog posts are being shared to Mastodon.
Great plugin, thanks!