Preheat oven to 350. Grease and flour a baking sheet.
Mix butter and lemon juice in microwave safe bowl. Heat until butter is melted. Stir in flax seed and set aside 3 minutes.
Cream sugar, butter mixture, and lemon extract in mixer using dough beater. Add baking powder, flour, lemon peel, and blueberries. Blend at medium speed until well incorporated. Add buttermilk 2T at a time until dough ball forms, pulling all materials onto the dough beater.
Divide dough into half and form into two flat logs approximately 2 1/2 inches wide by 10 inches long. Place onto prepared baking sheet. Bake for 30-35 minutes (until baked but not browned). Reduce oven heat to 325 degrees.
Cool logs on wire rack for 10 minutes.
Create desired slices from each log, cutting at a 45 degree angle using a serrated knife. Place back onto the baking sheet with a cut side up. Bake for 5 minutes. Flip biscotti to the other side, then bake again for 5 minutes. Cook on wire rack.