they must have fixed it cause it works now.
it's case sensitive so you have to type the subreddit name exactly, you don't need the r/ part, and also takes a second to load.

you will see a subreddit you thought you blocked but give it a second and it goes away.
it's good but it needs a button next to the subreddit name to block it. i would give it 5 stars if that feature was there.