Place the watermelon chunks into the blender jar.
Add the sugar (if using) and blend on high until the mixture is completely smooth and vibrant pink, about 30-40 seconds. Observe the silky texture and uniform color.
Pour the blended watermelon through a fine mesh strainer into a large pitcher, pressing gently with the back of a spoon to extract as much juice as possible. Stop once the pulp remains in the strainer.
Add the fresh lemon juice to the strained watermelon liquid and stir well with a spoon to combine. The mixture will turn a slightly brighter pink and smell citrusy.
Chill the lemonade in the refrigerator for at least 30 minutes until thoroughly cold. Serve over ice for a refreshing, juicy appearance with bubbles forming around the ice cubes.