Yeah, it's coming in v19. How do I know this? Because the instance I previously used decided to update to v19 RC1 despite being a production instance. It has instance blocking in the settings. Lemmy maintainers broke API interoperability for v18 and Boost doesn't support v19 yet, though. I decided I would rather use a different instance on Boost than use the web interface on that instance.
Mute is local client-side filtering, blocking happens server side.
When you mute something it's just filtered out of the view you're presented with in the client - you'll still see that content if you visit using a web browser.
v0.19 will have this feature natively and app developers can use the API endpoint to allow their users to block posts from whole instances from their feeds. It won't block comments however.
I'm missing a way to block posts that contain certain domains. With Boost for Reddit you could easily block posts with, let's say, facebook.com in it by opening the menu for that post. Right now it only lists block user and some other thing I forgot.