Prepare the syrup by combining 2 cups blueberries, 1½ cups granulated sugar, and 1 cup water in a small saucepan.
Bring the berry syrup mixture to a simmer over medium heat. Simmer for 10 minutes; the sugar should have dissolved, and the berries should be bursting.
While the simple syrup simmers, wash the 2 cups lemon juice, then slice them in half and use a citrus press to squeeze the juice out of the roasted lemons into a liquid measuring cup.
Remove the syrup from heat and allow to cool in the pot for 20 minutes, then strain through a fine-mesh sieve, squeezing the berries to extract as much juice as possible. You can also just use the whole recipe without straining, I love the pulp from the berries in this!
In a pitcher, combine the lemon juice, 2 cups blueberry simple syrup (you will have some leftover - make blueberry sangria with it!), and 8 cups cold water.
Slice 2 lemons into 8 slices and add 4 to the lemonade, then stir to combine.
Serve in a highball glass with ice and garnish each with a lemon slice. Serve with a straw.