Is there a way to see which communities/users you've blocked?

Only via the website. In Settings -> Blocked. It shows both blocked users and communities.

At least from the website, go to your user, hit the dropdown and go to 'Settings'. There will be a second tab at the top to show the 'Blocks' you put in place.

If you are in the browser version, go to Settings. There are two tabs there - Settings and Blocks

If you're an admin, you can get that info via SQL, otherwise I don't really know.