Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, sugar, baking powder and salt. Cut the cold butter into small cubes and add to the dry ingredients.
Starting at low speed, beat the butter and dry ingredients. Increase the speed to medium and mix until the butter breaks into small pieces. Then stir in the milk, lemon zest and juice, and vanilla until moistened.
Place the dough on a floured surface and fold in the blueberries, being careful not to break them. Form the dough into a long rectangle, about 1 inch thick. Cut the dough into 2 squares. Cut each square in half diagonally, and then cut those pieces in half again. You should have 8 triangles total.
Transfer the scones to the prepared baking sheet, spacing them 2 inches apart. Bake until golden brown, 17 to 21 minutes. Let cool. Serve with Honey Butter and Blackberry Jam.
In a medium bowl, combine the butter and honey and blend thoroughly.
To serve, transfer to a butter crock.
You can also roll the butter into a log, chill it and then slice it into individual pieces. Place the blended butter on a piece of plastic wrap, shaping it into a rectangle. Wrap the butter tightly in the plastic wrap, folding the sides in first and then the top and bottom. Roll into a log and refrigerate until firm. To serve, cut 1/4- to 1/2-inch slices and arrange them in a butter dish.
Place the berries in a large stainless steel or enamel saucepan. Bring to a full boil over high heat, mashing the berries with a potato masher as they cook. Add the lemon juice and boil hard for 1 minute, stirring and mashing constantly.
Add the sugar and return the mixture to a boil. Cook, stirring constantly, until it begins to look syrupy and thickens slightly, about 5 minutes. Remove from the heat and test for doneness: a candy thermometer should register 220 degrees F and the mixture should coat the back of a spoon. Run a finger--or a clean object about the width of a finger--through the coat of jam. If the jam does not run and fill the gap, it is done. Otherwise, return the pan to the heat for another 1 to 2 minutes and test again. Be careful not to let the mixture get too thick--it will thicken as it cools. (See Cook's Note)
When the jam is done, transfer it to a heatproof jar and cool to room temperature, about 2 hours. Cover with a tight-fitting lid and refrigerate. The jam will keep for up to 2 weeks.
The key here is not to overcook the jam. It cooks very quickly and chances are that if you think it is thick enough, it probably is. Always remove it from the heat to test for doneness, since the time you take to test it could cause overcooking. Better to err on the side of undercooking, as once it is overcooked--dark, thick, and smelling like caramelized sugar--it cannot be saved.