In a large mixing bowl, gently blend the blackberries with sugar and cornstarch/tapioca.
In another large mixing bowl, sift together the flour, sugar, baking powder, and salt.
Cut the butter into the flour mixture until mixture resembles coarse crumbs.
In a separate bowl, combine heavy cream with the beaten eggs.
Add the cream-egg mixture to the flour-butter mixture.
Stir just to moisten; do not over stir.
Pour berry mixture into a baking dish.
Bake at 400 degrees for 20 minutes to get the berries partially cooked and thickened. This prevents the biscuits from absorbing the juice before they're fully baked.
Remove the hot berries from the oven and spoon cobbler biscuits on top in nice-sized mounds.
Sprinkle the biscuits liberally with sugar.
Turn the oven temperature down to 375 degrees and bake for another 20 - 30 minutes on the lowest rack to prevent the biscuit topping from burning.