Country blacklists don't block spam, they block all mail from that country. Here's the idea.
You don't know anyone who lives in Angola. Someone adds your email address to a list, for a legitimate provider or service in Angola. But, just because it's a legitimate provider doesn't mean you want the mail, and you don't speak Portuguese, so you can't figure out how to unsubscribe. By blocking all mail from geographic regions where you don't have any correspondents, you can cut down on all types of unwanted messages.
Because it blocks all mail from that region, you should only turn on country blacklists for countries (or continents) where you don't know anyone. In the above example, if you had a penpal in Angola, that mail would also be blocked, even though your penpal is not a spammer. Also, Blackberry is headquartered in Canada, so blocking mail from Canada can cause you to lose mail from Blackberry users.