Line two baking sheets with parchment paper or silicone mats. Sift the powdered sugar and almond flour together three times into a large bowl, pressing any stubborn pieces through with a spoon. Set aside.

In a stand mixer fitted with the whisk attachment, beat the egg whites and cream of tartar on medium speed until foamy, about 1 minute. Gradually add the granulated sugar one tablespoon at a time, then increase to high speed and beat until stiff, glossy peaks form, 3-4 minutes. Add vanilla and food coloring in the last 30 seconds if using.

Add one-third of the dry ingredients to the meringue and fold gently with a flexible spatula using a scooping motion from the bottom. Add remaining dry ingredients and continue folding, pressing the batter against the side of the bowl every few strokes. The batter is ready when it flows like lava and you can draw a figure-eight without the ribbon breaking, about 50-60 total folds.

Transfer batter to a piping bag fitted with a 1/2-inch round tip. Pipe 1.5-inch circles onto the prepared baking sheets, spacing them 2 inches apart. Firmly tap each sheet on the counter 5 times to release air bubbles. Let shells rest uncovered at room temperature until a dry skin forms and they no longer feel sticky when gently touched, 30-60 minutes.

Preheat oven to 300°F (150°C). Bake one sheet at a time on the middle rack for 14-16 minutes, rotating halfway through. Shells are done when they no longer wobble when gently touched. The feet should be well-developed and the tops should be smooth and firm. Let cool completely on the baking sheet before removing.

For the buttercream, simmer 1/2 cup fresh blueberries with 1 teaspoon water in a small saucepan over medium heat until broken down, about 5 minutes. Strain through a fine-mesh sieve, pressing to extract all juice. Return juice to the pan and simmer until reduced to 3 tablespoons of thick puree. Cool completely.

Beat the butter in a stand mixer on medium-high speed until pale and fluffy, about 3 minutes. Reduce speed to low and gradually add the sifted powdered sugar, then the salt. Add the cooled blueberry puree and heavy cream, beating until smooth and light, about 2 minutes more.

Match macaron shells into pairs of similar size. Pipe or spread 1-2 teaspoons of buttercream onto the flat side of one shell, then sandwich with its partner, pressing gently until the filling reaches the edges. Refrigerate filled macarons in an airtight container for at least 4 hours or overnight to allow the shells to soften and flavors to meld before serving.