Adjust rack to center of oven, and heat to 400 degrees.
Place a Silpat baking mat on a baking sheet, and set aside. In a large bowl, sift together flour, 3 tablespoons sugar, baking powder, and salt. Using a pastry blender, or two knives, cut in butter until the largest pieces are the size of small peas.
Stir in blueberries and zest. Using 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 mix well.
Pat dough into a 6-inch square about 1 1/4 inches thick.
Using a floured knife, cut into four 3-inch squares.
Cut squares in half on the diagonal to form eight 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.