Once they've turned blue/purple there's not much you can do to undo that. The good news is, this won't hurt you cookies in any way! They'll just be a weird blue/purple color. The dough might also be a little stickier due to excess moisture, so you may want to mix in another 1-2 tablespoons of flour if the dough is hard to scoop. To prevent the cookies from turning blue/purple make sure that your oven is at a true 200F temperature (not higher than that). And be gentle when you're mixing — use the spatula to gently fold the ingredients together rather than mixing vigorously. You don't want too many of the blueberries to burst!