To make the blueberry sauce, place blueberries in a small blender or food processor. Blend until smooth. Combine blueberry puree, sugar, and water in a medium saucepan over medium heat. Bring to a low boil and stir slowly until sugar dissolves. Reduce to a simmer and cook, stirring occasionally, until mixture thickens to a syrup-like consistency, about 10-15 minutes. Remove from heat and set aside to cool completely. (Blueberry sauce can be made in advance and kept in the refrigerator in a sealed container for 1-2 days.)
To make the crust, preheat oven to 350°F. Stir together graham cracker crumbs and butter. Press into a 9-inch tart pan. Bake 10 minutes. Remove from oven.
To make the filling, whip egg yolks with an electric mixer until thickened and lighter in color. Add sweetened condensed milk and mix until well blended. Add lemon juice and lemon zest; mix well. Pipe or drizzle blueberry sauce in horizontal lines over the filling. Use a toothpick to swirl the sauce into the filling, moving in straight lines perpendicular to the blueberry sauce.
Return to oven and bake 15 minutes. Cool pie at room temperature for 30 minutes, then transfer to the refrigerator to cool completely. Garnish with blueberries and lemon slices. Store covered in the refrigerator.