Preheat the oven to 375°F and line a 9x9-inch baking dish with parchment paper and set aside.
Prepare the combining 2 cups blackberries, 1 cup granulated sugar, and 1 cup water in a small saucpan.
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 softening and breaking up.
While the simple syrup simmers, wash the 5 small lemons, then slice them in half and place them in the baking dish. (see notes)
Roast the lemons for 25 minutes.
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.
Remove from the oven and let the lemons cool for 10 to 15 minutes.
Use a citrus press to squeeze the juice out of the roasted lemons into a liquid measuring cup.
Strain the roasted lemon juice by pouring it through a fine-mesh sieve or use it as is for a pulpy lemonade. Cool to room temperature before using.
In a pitcher, combine the cooled roasted lemon juice, 1 cup blackberry simple syrup (you will have some leftover - make blackberry margaritas with it!), and 4 cups cold water. Slice 1 lemon 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.