The domain of the link is mastodon social though, the comment may have been made on a community hosted on eco.br but the user is from Mastodon. What you want is a user or status block not an instance block.
No, I'm pretty sure I want to block the instance that's shown at the top of the post in it's entirety.
When I hit block instance I expect to block the instance of the post I'm viewing from the user on eco.br.
Also, I really wish there an Undo link on the little banner at the bottom in case I block an instance on accident. In fact, all of the blocking actions with that banner should have an Undo function.
But the link in the post is to a status from mastodon.social, so if you just go on that link and say block instance the domain it's on is not the Lemmy site. You should be able to go into blocked instances manually and add lemmy.eco.br there though.
Under settings you can add it there directly.
A community for the mobile app Connect for Lemmy.