Heat oven to 400°F (205°C).
Line a large baking sheet with parchment paper.
In a bowl of a food processor, combine flours, zest, sugar, baking powder and salt. Add cold butter and pulse until the mixture looks like small pebbles. Stir in blueberries, then milk, mixing only until large clumps start to form. When it comes to baking, mixing just until the components start to come together yields lighter, fluffier and less “hockey puck” scones. Use your hands to reach inside the bowl and gently (VERY gently) knead the mixture into one mass. The more you knead, the wetter the dough will get as the blueberries break up, so work quickly and knead only a few times, if you can get away with it.
Transfer dough to a well-floured counter and pat into a roughly 1-inch tall disc. Cut into 8 to 10 wedges, do not fret if the blueberries are now making a mess of the dough; once it starts cooking in the oven everything will work out just fine! Transfer wedges to prepared baking sheet, spacing them apart. Brush the tops of each with egg, and then sprinkle with coarse sugar.
Bake for 15 to 17 minutes, until scones are golden brown on top. Serve warm.
If you aren’t going to eat all the scones at once, bake as many as you will eat and freeze the unbaked raw scones. To make sure they don’t stick together, put all the unbaked scones on a cookie sheet and place in your freezer. Once frozen, place them in a large ziplock bag. You can bake them right from the freezer, just add 3-5 minutes extra bake time.