Preheat oven to 350°F.

Immerse the peaches in boiling water for 30 seconds to 1 minute, until their skins peel off easily.  Place them immediately in cold water.  Peel the peaches, slice them into thick wedges, and place them in a large bowl.  Add the lemon zest, lemon juice, granulated sugar, and flour.  Toss well.  Gently mix in the blueberries.  Allow the mixture to sit for 5 minutes.  Spoon the mixture into 6 ramekins or custard cups.  Or spoon mixture into a pie plate or baking dish.

For the topping, combine the flour, granulated sugar, brown sugar, salt, cinnamon, and the butter in the bowl of an electric mixer fitted with a paddle attachment.  Mix on low speed until the butter is the size of peas.  Alternatively, combine ingredients in a large bowl and cut butter into mixture using a pastry cutter or pastry blender.  Rub the mixture with your fingertips until it’s in big crumbles, then sprinkle evenly over the fruit. 

Place the ramekins on a sheet pan lined with parchment paper and bake for 40 to 45 minutes, until the tops are browned and crisp and the juices are bubbly.  If using a pie plate, bake for 55 to 60 minutes.  Serve warm or at room temperature with a scoop of vanilla ice cream or whipped topping.