Maybe it was just buggy a couple weeks ago when I tried it, but I tried blocking feddit.de and it didn't work. I would still see magazines from that instance, I had to block them individually to make them go away.
I had a bunch of instances blocked a while ago (ex: yiffit.net) and it worked for a while. But then they started showing up again. Same for magazines and communities. I'm not sure what's going on...