Wash an apple and remove most of the core, leaving about 1/4-inch at the bottom. Remove a thin strip of peel from around the cavity. Place the apple into a deep microwave-safe baking dish or bowl. Sprinkle the inside with a little cinnamon and fill with frozen blueberries. Drizzle the syrup over the filling and top of the apple.
Cover the dish with lid or plastic wrap. Microwave until apple is tender, 3-5 minutes, testing with a fork after 3 minutes. Allow to cool for 5 minutes before serving. (Caution: Inside can be very hot, so do let them cool before eating.)
You can cook these in the microwave in about 5 minutes, but if that's not an option, place them in a baking dish, add enough water to cover the bottom, and bake at 375F for about 45 minutes.