Add the sugar and 6 tablespoons of water to a small saucepan; heat over medium heat, stirring constantly, until the sugar is dissolved. Cool to room temperature.
Add the cooled sugar syrup, remaining ½ cup water, lemon juice, mint leaves, ice cubes, and orange blossom water (if using) to a blender. Pulse a few times to break up the ice and then process until slushy.
Pour into 2 tall glasses, garnish with mint leaves if desired, and serve immediately.