the entire point of the blue food was that it was the only way sally could consistently rebel against gabe without getting her or percy punished. sally is not and never was submissive in the books. percy himself says sally has "a rebellious streak, like me." however, sally and percy were under threat of serious abuse from gabe and therefore had to pick their battles. blue food was sally's way of telling percy "i am here and fighting with you" and "there is hope in the world still."