There's not a lot of details on this addon and how it works, so it's hard to say if its working as intended, but for me it does not work well. I use the "new" design, and the addon doesn't do much there, so not sure if it was meant for the "old" design. I very occasionally see a post for a blocked sub disappear because of the addon, but very rarely, and most of the time it does nothing (meaning I see tons of posts from blocked subs while viewing r/all). There are a few other minor nitpicks, like it doesn't prevent you from adding duplicates to the block list. But overall, it's just not effective.