Like the reason people end up as a blueberry in the first place? It's best to suspend your disbelief, but I don't mind trying to delve into the specifics of it. I like to think of the gum serving as a catalyst for a shift in one's genes. The gum is already sending signals to the body that it's consuming food, but the dessert causes a reaction that makes the blueberry part leeche to a person's cells and permanently switches their completely normal animal/human cells to being a weird hybrid cell that includes aspects of plant cells. In addition, it causes swelling based on their lipocytes becoming overfilled with juice. For the consumption of tainted juice by others, it only induces the swelling and color change, but it's not a permanent genetic change (Ex. Hailey and Yancy.)