It turns out I had to make a correction because I had to rewrite the conversation between Caroline and Stonewall. Technically Concord is under blockade since the start of the game, but Stonewall talks about it in a way that it only started after the battle. I also noticed another error because the battle on Concord is referred as a consequence of Plymouth being destroyed. But that means by the time you finish Concord Plymouth actually has to be destroyed. This way the variable triggers when you go to Concord, so it's taken care of.