My opponent had on the battlefield a Swarm Guildmage and three other creatures. My opponent then enters Combat and declares his three other creatures as attacking.
After this happens I then declare two of my creatures as blockers, each blocking one of his creatures, to which then he responds by activating Swarm Guildmage's ability that grants each of his creatures Menace. His argument is then that, because now his creatures have Menace, my blocks are illegal and I have to re-assign blockers.
My assumption is that once blockers are declared then no matter what happens afterwards during combat the blockers initially declared would remain legal.
Is this true, or is it that my opponent was correct and my blockers suddenly became illegal after the activation? If so, are there any other scenarios in which a blocker can be made illegal after being declared as a blocker?