Preheat oven to 425°F. Grease a cookie sheet with some additional butter.
Whisk flour, baking powder, granulated sugar, salt and cream of tartar in a large bowl.
Remove wrapper from ½ cup butter and grate, using the large opening of a box grater, onto a plate. It's best if this is cold from the refrigerator OR has been frozen ahead of time. This makes cutting in the butter so much easier.
Add butter to dry ingredients and use a pastry cutter or fork to cut the butter into the flour. If you’ve grated your butter this won’t take very long; it should be sandy looking. Add the zest and stir with a fork.
Add the buttermilk and stir with a fork until you get a shaggy dough. Add the blueberries and continue to fold carefully, adding up to an additional 2 tablespoons buttermilk until the dough just comes together. Some of the blueberries might break during this process, that’s okay.
Use 2 spoons to scoop 8 biscuits on the prepared cooking sheet. Melt remaining 1 tablespoon butter and brush over biscuits, sprinkle with granulated sugar.
Bake for 15-20 minutes or until they just turn golden on the bottoms and some of the little points on top turn golden. Cool 5 minutes then remove from cookie sheet (the heat of the cookie sheet might make them soggy). Serve warm or room temperature with butter.
Store in an airtight container for up to 2 days.