I think what most likely happens is:
Computer automatically scans shapes for prohibited items. Your blacklisted address comes up and the agent carefully looks at the screen and declaration as it passes through. If he/she isnt satisfied, then the package is opened. If there is no prohibited item, then the package is let go. An automated system can store a whole lot of addresses and can automatically sort through. Not to mention, all the blacklisters dont order stuff everyday so the system isnt overloaded.