Preheat oven to 400° Fahrenheit. Use cooking spray to lightly grease a 9x13 baking dish.
In a large bowl, use a fork to mix together dry ingredients. When mixed, grate frozen butter into bowl. Using same fork, lightly toss butter in dry mixture until butter is fully coated and evenly distributed throughout all of the dry ingredients.
Pour in 1/3 of buttermilk. Use fork to mix in. Repeat until all buttermilk has been used. Dough will be very soft and still wet.
Turn out dough onto a floured surface. Lightly dust top of dough with extra flour. Fold dough in half 2 times- right over left, left over right. If necessary, use extra flour from the floured surface to sprinkle on sides and top to keep dough from being sticky. Gently pat dough to make a 1 1/4 inch thick rectangle. Do not knead or roll.
Evenly place 1/3 of the blackberries (without juice) across the surface of the dough. Fold dough in half- right over left- so that berries are now inside the dough and pat lightly around top and sides (do not press, knead or mash) to make new 1 1/4 inch rectangle with blackberries inside. Repeat- alternating right over left, left over right folds- until all berries are incorporated into the dough adding extra flour to the floured surface as needed to keep dough from sticking.
When all berries have been incorporated, turn dough completely over. Gently pat sides and ends to close edges. Make sure dough is 1- 1 1/4 inch thick. Cut out biscuits using a 2 1/2- 3" diameted biscuit cutter. Fold leftover dough and make new rectangle and cut out biscuits until all dough has been used.
Place biscuits close together in greased 13X9 with outside biscuits touching the edges of the dish. Bake 20-25 minutes or until tops and side are golden brown.
While biscuits are cooking, make glaze by whisking together confectioner's sugar, lemon juice, zest and melted butter. Use a pastry brush to coat tops of biscuits and serve.
To heighten the berry taste, put berries in a small bowl, sprinkle with lemon juice and 1 tsp of sugar. Let sit in lemon juice and sugar mixture while preparing the dough. Drain away any juice from the berries and use as directed in the recipe.