This is specifically defined in section 13 of the official rules:
13.1.1 All actions which direct the ball towards the opponent, with the
exception of service and block, are considered as attack hits.
Your set directed the ball towards the opponent, therefore it is an attack hit and therefore it can be blocked. You may also wish to refer to 14.3:
14.3 In blocking, the player may place his/her hands and arms beyond the
net, provided that this action does not interfere with the opponent’s
play. Thus, it is not permitted to touch the ball beyond the net until an
opponent has executed an attack hit.