Block those who have blocked you
Now that individual unblock is active it's possible for people who you've chatted with to temporarily block you, and then quietly much later unblock and have all your chats and photos back without you knowing. There should be an option that if someone who you've chatted to blocks you then you can double block to prevent them secretly getting all this back.
Paddy C commented
I'm pretty sure the chat history is cleared when you block and unblock someone? Certainly seems to be the case from my end.
Mutual blocking is required, one person can't get the undue advantage of blocking and being able to see the person in his blocklist
When someone blocks your profile you can have the possibility to block him too.