Preheat oven to 425°F. Line a baking sheet with parchment paper or a silpat mat.
In a large mixing bowl combine the flour, baking powder, 2 tablespoons granulated sugar, salt and lemon zest.
Add 1 cup of the blueberries to the flour mixture and toss with a rubber spatula until blended.
Make a well in the center of the dry ingredients and pour in 1 cup of the buttermilk and melted butter.
Add the additional ¼ cup buttermilk if needed. Use the spatula to fold the ingredients together making sure you don't smash the blueberries. Mix just until all flour is moistened. If the dough is very sticky, add more flour 1 tablespoon at a time (up to 1/4 cup) until it holds together without being wet.
Gather the dough in your lightly floured hands and gently mold into a ball, Transfer to the prepared baking sheet and gently form into an 8x8 inch square, about 1-inch thick.
Flour a large knife or pizza cutter and cut the biscuits into 9 or 12 pieces. Do not separate the dough but leave them attached to one another.
Brush the tops of the biscuits with milk and sprinkle with coarse sugar if desired.
Bake in the preheated 425°F oven for 23-30 minutes or until golden brown.
Remove and allow the biscuits to cool while preparing the sauce.