The results were clear. No notification. No alert. No “Lena has unblocked you” message in his chat. Snapchat doesn’t send one. All that happens is his name quietly reappears in her search, and his stories become visible again—provided he’s still public or hasn’t blocked her back.
While there's no official alert, an observant user can spot "tell-tale signs" that they are no longer blocked: can someone see if you unblock them on snapchat
: If they try to message you, the message will no longer fail immediately (though it may stay as "Pending" until you re-add them). Key Effects of Unblocking The results were clear