For the mint syrup:

1. Combine mint, sugar, and water in a small saucepan. Bring to a boil and simmer until sugar is dissolved. Let cool.
   

2. Strain into a container with a lid. Chill well before using!
   

For the mojito:

1. In a pitcher filled with ice, combine blackberries and granulated sugar. Using a muddler or wooden spoon, crush berries enough to release their juice but not so much you turn them into jam.
   

2. Divide blackberries between 4 glasses and add 2 tablespoons of simple syrup and 2 tablespoons of lime juice to each glass.
   

3. Fill glasses with ice and top off with seltzer, then garnish with mint.