In which it would also be used to blacklist all the players who're starting doing PvP because they're not skilled enough. Which is something that is currently happening in some PF groups in which some players get blacklisted even without a warning or something because they're not good enough. Someone even went too far to the point they shouted in the chat: "Blacklist X player!!!" without specifying why and I have seen this more than once.
If Blacklisting players would stop them from being matched with you in DF then this may results in a longer queue time from either your end or theirs, which can be avoided by just ignoring them and not replying to any of their messages. The game shouldn't punish others and stop matching them with you because you just disagreed with them. In fact, it's ultra-rare for you to be matched with someone on your blacklist in any duty roulette unless one of you is spamming a duty roulette and the other queued for it then here there is a higher chance of getting matched up but otherwise its ultra-rare. As for PvP, if I'm not mistaken you can't chat with others. So in the end there is no reason to stop someone from being matched with you in DF. It can only cause harassment ( Mass/Encouraged Blacklisting on a player )
As I have said earlier, unless a player is a snowflake who literally blacklist anyone they disagreed with, I don't see how a player can have more than 50 other players in their blacklist without counting the bots.