Preheat oven to 350 degrees

Mix peaches, blackberries, 1/4 cup sugar, cinnamon, and lemon juice in a bowl.

Pour butter into a 9x13 baking dish and evenly coat the bottom and sides. Whisk flour, 1 cup sugar, baking powder, and salt in a bowl. Mix in milk, stirring just to moisten dry ingredients. Pour batter into prepared baking dish and distribute fruit mixture evenly over the batter.

Bake in preheated oven until cobbler is golden brown, about 45 minutes