Better to make it a whitelist in case somebody no longer wants to receive messages from somebody they've corresponded with in the past. Otherwise if they've ever corresponded with a troll or have had a falling out with friends then the filter is ineffective.

The more I talk about this the more it sounds like a score file[1]. Obviously it would need to be made more user-friendly to be really effective.

[1] http://slrn.sourceforge.net/docs/slrn-FAQ-4.html