Because you blocked them previously, your chat history has been wiped. Even if you add them back:
: Occasionally, Snapchat may prevent you from re-adding a user immediately after unblocking them. If they do not appear in search results, you may need to wait 24 hours before they become searchable again. how to find someone you unblocked on snapchat
Finding them doesn’t mean they’re your friend again. To reconnect: Because you blocked them previously, your chat history