If their profile photo was previously a default gray silhouette and suddenly their actual picture reappears, it is a strong sign you are unblocked.
"WhatsApp Alert: [Contact Name] has unblocked you. You can now send messages." if someone unblocks you on whatsapp
Keep in mind:
If you are looking for a feature description for an app design or a user request (perhaps for a modded app or a feature pitch to WhatsApp), here is how that feature would logically function: If their profile photo was previously a default