Adjust rack to center of oven and preheat to 400 degrees F. Place a silpat (a French nonstick baking mat) or parchment paper on a baking sheet; set aside.
In a large bowl, sift together flour, 3 tablespoons sugar, baking powder, and salt. Using a pastry blender, or 2 knives, cut in butter until the largest pieces are the size of small peas. Stir in blueberries and zest.
With a fork whisk together cream and egg in a liquid measuring cup. Make a well in the center of dry ingredients, and pour in cream mixture. Stir lightly with fork just until dough comes together. Turn out onto a lightly floured surface, and knead a few times to combine.
Pat dough into a 6 by 6-inch square about 1 1/4 inches thick. With a floured knife, cut into 4 (3-inch) squares. Cut squares in half on the diagonal to form 8 triangles. Transfer to prepared baking sheet. Brush tops with cream and sprinkle with sugar. Bake until golden brown, 20 to 22 minutes. Transfer scones from baking sheet to wire racks to cool.
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