In a small pot, combine 1 cup of sugar and 1 cup of water. Bring to a boil and stir until the sugar is dissolved (3–5 minutes). Let cool completely.

Rinse 1 cup of fresh blackberries under cold water. In a blender, combine blackberries and 1 cup of lemon juice. Blend until smooth.

Set a fine sieve over a large pitcher and pour the blackberry mixture through it to strain. Press down to extract juice, leaving seeds behind.

Add 1.75 cups of cold water to the pitcher with strained juice. Stir well, tasting to adjust flavor if needed.

Gradually add the cooled simple syrup, tasting as you go to achieve desired sweetness.

Fill glasses with ice and pour the lemonade over. Garnish with blackberries and mint leaves before serving.