I was thinking a bit over the weekend about modifying Jay Allen's awesome MT-Blacklist plugin to work with Blosxom. I just did some googling, and searching of the Yahoo Group for Blosxom, and came up with Doug Alcorn's BlogSpam.
It's based on the MT-Blacklist plugin, and uses the same blacklist.txt file. It even merges in email notification of new writebacks, making my own wbnotify_nosendmail hack unnecessary and redundant.
If you're thinking of using Blosxom as a weblog in a public space, do yourself a favour and grab BlogSpam. It replaces the default writeback plugin, and only requires some minor customization. (I added some additional stuff to quote the name in the sender's email address, and to use a specified mailserver rather than localhost - I'll probably do some other minor modification to tweak the output message as well).
One thing I couldn't get working as per Doug's instructions was the automatic update of the blacklist.txt file using his perl scripts. It's easily cronnable without using Perl, though... I'll likely just have it curl the latest blacklist.txt file into place once a week or so...