There are cases where we want to block a player, for reasons that can’t really be solved by moderators (for example, covert strong players who play unranked with weak players to toy them around without rank repercussions).
I understand automatch doesn’t let you match with blocked players, so that’s something.
However, in the list with open challenges they are still shown. Since it’s not really easy to remember random nicknames, I may accept the open challenge from a person I really don’t want to play against.
In the meantime you can accept the challenge, then click on the username to check if your opponent was blocked. If so then you can cancel the game immediately.