Cream: Bring cream and lavender just to a boil in a small saucepan. Remove from heat, cover, and let cool completely. Refrigerate until well chilled, up to 2 days.
Crusts: Pulse flour, cornmeal, sugar, and salt in a food processor to combine. Add butter and pulse until mixture resembles coarse crumbs. Add yolk and ice water and pulse until dough comes together, then knead until very smooth and combined. Pat into a disk, wrap in plastic, and refrigerate at least 30 minutes or overnight.
Let dough stand at room temperature until pliable. Roll out on a lightly floured surface to a little more than 1/8 inch thick. Cut out 4-inch circles, gathering scraps and rerolling dough once. Fit into twelve 2 1/2-inch-diameter tartlet pans, making sides thicker than bottoms. Place pans on a rimmed baking sheet and freeze until firm, about 30 minutes.
Preheat oven to 350 degrees. Bake on baking sheet until golden brown, pressing down often on centers as they puff up, about 25 minutes. Let cool completely.
Filling: Pass cold lavender cream through a fine sieve into a mixing bowl. In a small bowl, mix sugar and vanilla-bean seeds with fingers until combined. Add to cream and whisk with a mixer on medium speed until soft peaks form. Dividing evenly, dollop cream into tart shells, top with blueberries and lavender, and serve.