The spam filter address blacklist just affects the spam filter score, it does not automatically block messages.
So, if there is some other spam filter rule which affects it the other way (eg the address or word whitelist), that could counteract the blacklist. Or if the spam filter quarantine is disabled, either globally or for that user, then no messages will be quarantined, even if the message receives a very 'spammy' score from the spam filter. Or, maybe there's a misspelling somewhere, in either the blacklist entry, or in the email.
You need to look at the full message headers, especially any lines beginning with 'X-VPOP3....', as those will let you see what the spam filter thought of the message. The X-VPOP3-Spam header will give the scores that the various rules assigned to the message, but the other X-VPOP3 lines may give more information on particular parts.