To unblock a user, you need to only send them a message. They may have blocked communications with you if you are still having issues.
If you cannot connect to parties and you receive the message "Cannot Connect to Xbox LIVE Party" - that is a connection issue between you and the user, not a result of you blocking communications.
Also, try checking your NAT status, or verify that you both have the appropriate privacy settings.
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how can i unlbock though
All you have to do is send the person a message and that will unblock them.