Mix cider and milk together in large saucepan and heat. Curds will form, stir them down. Just before mixture reaches a boil stir in oats. Cook just shy of a boil for 10 minutes until oats are soft, but not mushy. Stir in frozen blueberries and salt gently. Reduce heat to warm. Let cook 10 more minutes, stir as little as possible so blueberries maintain shape.