Correct, but you're better off to wait to block then as they come to the watch though. That's much quicker. In the companion app, you have to tap the +, then select the app from the list of all of your apps & repeat for each app you wanna block, where it's just an extra swipe to the side on the watch when it has a notification.
AFAIK, the only way to unblock apps is through the companion app. That's just a matter of going into the list of blocked apps & clicking the X by any one you want to unblock.
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