Personal preferences vary, but I very rarely elect to actually blacklist someone. They pretty much have to be flooding chat in a channel I have no direct control over (like shout). And even then I'd just temporarily blacklist them until a GM deals with them and prevents recurrence.

However, there are many people I encounter that prove to not be a good fit for my parties, whether it be because they're mean to people, etc. Yes, such things are often actionable by GMs ... but often they are too minor to actually warrant GM action.

Obviously, I can just tell them this when they join such a party, but such is very time-consuming, extremely confrontational, and in itself borderline harassment. As such, this suggestion is for the ability to maintain a list of players that party finder (and, ideally, duty finder, while we're enhancing things) prevents from joining my party without having to completely deafen myself to anything they ever say to anyone in any channel I can see.