Tart apples marry well with sweet, spicy blueberries, even though their seasons are fairly opposite. In early summer when blueberries come into season there are always imported apples available, and when apples come into season it’s okay to use frozen blueberries. I find that dicing the apples quite small makes them cook through easily, and their tartness gives a better boost to the blueberry flavor than adding lemon juice or zest.