Add the sugar and 3/4 cup of water to a small saucepan and bring it to a boil over medium heat. Once it starts bubbling, lower the heat and let it simmer gently without stirring for about 3 to 4 minutes, until the sugar has fully dissolved. This slow simmer keeps the syrup clear and smooth instead of grainy.
Take the syrup off the heat and stir in the black gel food coloring a little at a time. I like adding it gradually because gel coloring is concentrated, and a small amount goes a long way toward that deep, inky black shade. Let the syrup cool completely before moving on.
In a large pitcher, combine your fresh lemon juice with the cooled black syrup. Give it a good stir so the color distributes evenly through the juice before you add any water. This step is where your black lemonade recipe really starts coming together visually.
Pour in the 4 cups of cold water and stir everything together until it's uniformly black and well combined. If you have time, pop the pitcher in the fridge for 20 to 30 minutes so it's extra cold before serving.
Fill glasses with ice and pour the lemonade over the top. Give the pitcher a quick stir before each pour since the color can settle slightly, and serve immediately for the best presentation.