It will never happen. It would make matchmaking take forever and become very difficult, or even impossible.
Merrykitten is right on this one.
If you have a feature where you have a list of players who won't be put into your game at all, it's worse than even that situation. The first thing that is apparent is that since you would only be blacklisting people you are matched with, you would only be blacklisting people of your skill level, making the matches be worse for you, being matched with those above your level or worse for those below. There's more.
Say you have been playing for six months and have 30 people blacklisted (one a week. That's probably too conservative an estimate if we think about it). Not only is your list limiting matches but each subsequent person's list adds to it. 5 player's in, that's 150 people of that MMR that are disqualified from being placed in that game. Not only that, the 6th player's blacklist can't match anyone placed in the game and applies to his potential matches who could then be then compared to him, you, your game matches, and their potential matches. And we know people are going to have more than 30 on their lists.
But wait, it gets worse! That's just your blacklist. A single player could be on a hundred players' blacklists, even when they are do not behave poorly. That's not a huge estimate; you play poorly on a new champ... you talk back a bit too much to a teammate complaining in allchat... your client DCs in an ARAM and you can't get back before your team loses... BOOM, you could get 20 blacklists right there. Even if you never did anything jerkish even once. We don't even need to consider the amount of people who would blacklist from something minor, "I don't care about how you had dragon fights, you never ganked my top. Blacklist."
