This isn't super intuitive or really mentioned when you block them so, this is a PSA (or if you already knew, a reminder/news to people other than you). It doesn't show up on the info page for blocking but it is in support documentation, by which I mean I saw a user-generated response about it on the support site.
Edited for clarity: Blocking does stop them from continuing to comment or to create other notifications for you—it simply doesn't hide notifications that they triggered before you blocked them. So let's say you get a notification that someone commented on a post, you see that it's rank, so you block them. Now, they can't continue to be grody, but you'll still see the full text of their initial rancidity every time you click on notifications until either the notification ages out or until you silence them in addition to blocking them.
To be clear, I do wish it either worked intuitively, was signposted when you block someone, or at the very least mention made on the help page for blocking, BUT I'm NOT attacking the devs about this. I am just sharing something I'm sure many other people have also shared many times, because I myself literally only just learned today.
Do not start discourse about this, I swear to fucking bug