If you report someone post-game, why not make it to where they are only temporarily blacklisted?
One of the most bothersome problems with Ranked is how you can requeue with people who just flamed you all game. It's not a fun experience, at all.
The main reason that people say why they cant blacklist is if you do that in high elo, you wont have anyone to play with.
So, why not do a "soft" blacklist? If you report someone from post-game, have matchmaking prefer to avoid that match-up for the next game that you play.
In lower elos, there's so many ppl that you wont ever see him again. In higher elos, you'd have to report the same people, again and again, in every match you play, and even then it would only make it lower priority, so attempting to game the system would only result in something very suspicious.
How about it? The biggest problem is not blacklisting, but avoiding a certain person who is extremely hooped up on rage towards you. I can't say how many times ive had to report people twice, simply because the second time they decide to "teach me a lesson." Would be a great amount of stress relieved if you could avoid people instead!