I think there’ll be an error message if you try to text the person who blocked you. You can still most probably see their activity status and profile like someone who blocked you can see yours too. Not sure though, because I haven’t ever been blocked on the app. You’ll be able to see their messages in a server though, and they can too.
If you send a DM to someone who has you blocked, it will be in red text and will tell you that you can’t send messages to that user.