Matched with a player on my Block List?

I don't get it, what's the point of a block list if I can get matched in to games with a player on my block list? I never noticed until after the game when I was looking to honor someone and noticed someone had a grayed out name and then realized it was someone I had blocked previously.

This is so bad. The whole game I never see anything he says, meaning I couldn't react accordingly if he was asking for help or anything (who knows if he improved to a more reasonable player). It also doesn't help that he went 1/6 in lane (the 1 being when I roamed from mid when he was 0/5 and netted him a kill) so I feel cheated because one of the players that hurt our team was someone I should never have been matched with in the first place (also, seriously, 2 players (bot lane) on my team didn't connect on the loading screen and it still started the game and they were 4 levels behind when they finally connected, why didn't it wait for them to load?). Back to topic though, why was I matched with someone I had blocked? Is there any way to prevent getting matched with someone that you didn't like playing with in solo queue?