Make the crust: With an electric mixer on low speed, beat butter and 1/4 cup sugar until just combined, about 15 seconds. Add flour and salt, and beat until dough comes together, about 3 minutes. Shape into a disk.
Pulse remaining 1/3 cup sugar and the almonds in a food processor until finely ground. Scatter half of the almond mixture onto a clean work surface, and place dough on top. Scatter the remaining almond mixture on top, and roll out dough, resprinkling with almond mixture and incorporating into dough, until dough measures an 11-inch square.
Preheat oven to 325 degrees. Fit dough into a 9-inch square tart pan with removable bottom, pressing into bottom and up sides. Trim dough flush with rim. Refrigerate until firm, at least 30 minutes (or up to 1 day).
Bake tart shell, occasionally pressing bottom to flatten using the bottom of a glass, until golden brown, about 30 minutes. Let cool.
Make the filling: Sprinkle gelatin over cold water in a small bowl; let stand until softened, about 5 minutes.
Heat cream, sugar, and salt in a small saucepan over medium heat, stirring to dissolve sugar and salt. Add gelatin mixture, and stir until it has dissolved. Remove from heat; let cool. Stir in buttermilk and lemon juice.
Remove tart from pan. Spread custard into crust. Refrigerate until custard is slightly set, about 15 minutes. Scatter blueberries evenly over top. Refrigerate until custard is firm, about 2 hours (or up to 1 day). Cut into squares to serve.