If they changed their username while you had them blocked, you will not find them. Snapchat allows username changes every 365 days, and you were literally blind to their account during the block. You’ll need to ask a mutual friend what their new handle is.
Finding a person you just unblocked on Snapchat can feel like a disappearing act. Once you hit that unblock button, the app doesn't automatically put them back on your friend list. They vanish from your blocked list, but they don't reappear in your chats. how to find someone you just unblocked on snapchat