Place the apples and kale in a steamer basket set over boiling water. Cover and steam 8 minutes. Add the blueberries and steam an additional 2 minutes (optional).
Puree with an immersion blender or food processor until smooth, adding a small amount of the cooking liquid if needed. If desired, strain the puree through a fine-mesh sieve. Cool and serve or store.Purple power puree can be stored in the refrigerator in an airtight container for 3 days or in the freezer for 3 months. If freezing, spoon individual portions into ice cube trays or baby food containers, then cover and freeze.