In a large mixing bowl, combine the sugar, cinnamon, salt, and cornstarch. Add the peaches, and blueberries, and mix, coating the fruit evenly with the sugar mixture. Toss with the lemon juice and zest, if desired. Put the fruit filling in a cast iron skillet and place in the oven for 15 to 20 minutes. In the meantime, make the biscuit dough.
For the topping:
Whisk the flour, baking powder, baking soda, sugar, and salt in a large mixing bowl. Using a food processor, a pastry cutter, or your fingertips incorporate the chilled butter into the dry ingredients until the mixture resembles coarse meal. Add the buttermilk and stir just until moistened.
Carefully remove the skillet from the oven, and drop 6 to 8 (1/2 cup) portions of biscuit dough on top of the cooked fruit, spacing the mounds about a 1/2-inch apart.
Increase the oven temperature to 425 degrees F and bake until the biscuits are golden brown on top and the fruit is bubbling, about 30 minutes. Allow to cool for 15 minutes before serving.