Dry the blueberries: Preheat oven to 200°F. Arrange blueberries on lined sheet pan. Place on middle rack for 2-4 hours, shaking pan occasionally. Blueberries shouldn't burst, but will wrinkle and shrink slightly. You aren't turning them into raisins, just removing about 25% of the water content.
Melt, then cool the butter: Melt butter in 15-20 second bursts in a large microwave safe bowl. Let cool for 20-30 minutes, until bowl is cool to the touch and butter has just barely started to solidify around the edges.
Whisk: Add both sugars and whisk vigorously, 1-2 minutes. The mixture should be lighter in color, have a slightly gritty, paste-like texture, and will hold its shape when you stop whisking. Then, add the egg, vanilla extract, and orange zest. Whisk vigorously for another 1-2 minutes until very pale, smooth, and glossy.
Stir: Add the flour, salt, and baking soda. Switch to a spatula and stir until dry ingredients are almost completely incorporated. Then add blueberries and chopped dark chocolate and gently fold until evenly incorporated and no dry bits of flour remain in the bowl. Try not to burst any of the blueberries.
Scoop: Use a 2 tablespoon cookie scoop to scoop the cookie dough onto a lined sheet pan. This can be a little fiddly so just be careful as you scoop to make sure you're getting an even distribution of blueberries in every scoop. The blueberries might burst a little (that's okay) and you want to make sure you don't accidentally scoop a cookie that is mostly blueberries or it won't hold together in the oven.
Chill: Cover cookies and chill in the fridge for at least 2 hours (ideally overnight) prior to baking. You can even freeze the dough balls and bake from frozen.
Bake: Preheat oven to 375°F. Arrange cookies on lined sheet pan with about 2-3" between them (I can usually fit 10 cookies on a sheet pan). Bake for 9-11 minutes, until edges are browned and tops have mostly set. Top immediately with flaky salt. Let cool on sheet pan 5 minutes, then transfer to a cooling rack to cool completely.Optional flair: To get perfectly round cookies, use a round cutter or overturned glass to scoot them into shape while still hot out of the oven.