After a few more new users complaining, I think the problem is the timing between the user posting a new message with a link, and an admin approving their message. Since we only have one admin, during that time window, the new user gets a message from Discourse that their account has been blocked, which creates a very negative first impression.
I would like to suggest softening the message from “Account temporarily blocked” to something along the lines of
“Thank you for joining our community! To keep spammers out, we’re reviewing the first message posted by new users. If you’re not a spammer, we’ll approve your account shortly.”