In a medium sauce pan, dissolve the cornstarch in the cold water. Add the blackberries, sugar, lemon juice, and lemon zest.
Bring the berry mixture to a boil over medium-high heat, then reduce heat until the berries are simmering. For fresh berries, simmer covered for 20 minutes. For frozen berries, simmer covered for 10 minutes. Transfer the cooked berries to an 8 inch square baking dish or a 1½ quart oval baking dish (pictured in recipe post). Set aside.
Preheat the oven to 400°F.
In a medium mixing bowl, combine the flours, sugar, baking powder, and salt. Mix with a fork to combine. Cut in 4 tablespoons of chilled and cubed butter and distribute the butter using a pastry cutter. Work the butter in until small pebbles appear in the flour mixture. Stir in the boiling water and mix to incorporate.
Turn out the biscuit dough on a large sheet of parchment paper. Break the dough into small pieces and flatten with your palm to form rough round biscuits about ½ inch thick. The dough will be slightly sticky, so you may need to refrigerate a few minutes before forming.
Evenly place the biscuits on top of the berry mixture. The biscuits should cover the majority of the berries. Bake the cobbler 20 to 25 minutes, or until the berries are bubbling and the biscuits are lightly golden brown. Note that depending on your oven, this cobbler may take up to 40 minutes to cook. Remove from heat and let it cool for about 20 minutes to allow the berry filling to thicken. Serve alone or with your favorite ice cream!