How to add custom blocks?

From what I can tell, the default web interface doesn't let you quickly add a bunch of items to the block list. However, you can still block them by selecting the three dot menu on a particular comment or post. I just tested it out, and I was able to block users, communities, domains (the site that a link is from), and instances.

You can then go to settings > blocks and look under this section to unblock items:

Manage what users, communities, domains or instances you want to block. Blocking them means you will no longer see any posts associated with them.

A different app or future custom frontends might make it easier. In the meantime, you could request the functionality you're looking for by posting in !piefed_meta@piefed.social

Is it disabled on piefed.ca specifically?

Nope

Most people here use the mobile apps like Voyager, Raccoon which makes the blocking system more streamlined.

Gonna be honest I don't use the website/webapp much except for mod/admin use so I didn't even realize the blocking system wasn't fully fleshedout on the web frontend.

Also Welcome to Piefed.ca/the Fediverse!🤗🌻