Preheat oven to 375 degrees F. In a medium bowl combine the first six ingredients (through salt). Add butter to flour mixture. Beat with mixer on low until pea-size crumbs form. Add cream all at once to flour mixture. Stir just until dry ingredients are moistened.
Turn dough out onto a lightly floured surface; knead dough by folding and gently pressing it for 3 to 4 strokes, just until dough comes together. Pat dough into an 8-inch square. Cut square in half diagonally each way. Cut each quarter in half to make a total of 8 triangles. Brush with beaten egg. If desired, sprinkle with additional sugar.
Grease a 2-qt. square baking dish. In a large bowl combine 3 Tbsp. sugar and the cornstarch. Add blueberries and liqueur and toss gently. Transfer to prepared baking dish. Arrange scones on top of blueberries. Bake 40 to 45 minutes or until berries are bubbly and scones are golden brown. Cool slightly; serve warm.