Hi Johnny,
Deleting the "Invalid Recipient" record will not unblock them. There is a list on our side once they are blocked. If you are sure this person would get emails again you can open a support ticket with that person's email address listed and the URL to your CRM. (http://support.clickdimensions.com).
It is possible that someone else on your team had that person unblocked. Or that the email address used to be incorrect and someone fixed it. We will go based off of the email address not the contact. So if the email address was changed to be correct, it would try to send to them again next time.