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Blake and his friends were in the frontlines, due to Blake's commands, they had yet to fight themselves, and were not joining in the phase 1 of the fight for obvious motives, they needed to save their strength, mana, ki, and stamina for phase 2, where things would finally get real.
Nonetheless, it wasn't as if Blake wasn't helping in other ways, as he had used his Commanding Skill and his Cooking Skill magisterially, combining both buffs by feeding most of the people here with his cooking and also commanding them with Lukas help. Although they would never listen to a kid, if he was near Lukas, someone that was now promoted as a Knight Commander, they would listen.
Check "The Protagnist's Sister Is Actually the Strongest"!