One, it's an unfair way of determining an AFK/Leacher, as "block" is used for many reasons - either being harassed/annoyed or simply blocking spammers. So the limited block space will ultimately stop the mass blocking of spammers in world, peer, etc. Unless a mechanism is developed specifically for Arena blocking, but such system wouldn't be a proficient way of determining an AFK/Leacher from lets say a player that's better than you or offending you in some way or form.
Random que - this will have to be revamped. This system will have to go through people who are queing blocked list, that alone will make time que longer - along with making sure that xxxxxxx player doesn't que into your match or on your side - plus that other player queing in your match, and that other player, AND that other player is accounted for. Lets say due to you blocking that "suppose AFK/leacher" now the AFK is on the other side, how does that make the opposition feels? How about there isn't enough players in the que? Will the system than override the action and still allow that person in your block list to join either side or in your arena?
In my mind, It's more of a way of queing in players that are appropriate for you and not for the actual cause. Hence trolls/others can ruin time ques because they either not allowed that player on their side or in Arena, making that falsely banned user que longer, and most likely effecting other ques.