The harsh truth: If you deleted the message thread and cleared your recents, that number is gone from your iPhone. Apple assumes if you erased all evidence, you don't want to unblock them. Your carrier might have a record, but your phone won't.
Keep in mind that if someone has blocked your number, you won't be able to see if they're online or if they've read your messages. how do you find block numbers on iphone
This is the cleanest view. It shows names, not raw digits. The harsh truth: If you deleted the message