To make the blackberry ice cubes; place blackberries in an ice cube tray and fill the tray with water. Freeze until solid.

To make the blackberry syrup; combine blackberries, sugar, and water in a small saucepan. Place over medium heat and stir to dissolve sugar. Bring to a gentle boil and cook until blackberries start to break down, about 10-15 minutes.

Transfer to a fine mesh sieve set over a bowl and mash berries through the sieve to collect all the juice. Discard seeds. Let syrup cool.

To make the margarita; combine syrup, lemon juice, lime juice, tequila, and Grand Mariner in a pitcher. Stir to combine. Cover and chill until cold.

Rub a lemon slice around the edge of each glass and dip the rim in salt. Fill each glass with blackberry ice cubes and pour margarita into each glass. Garnish with lemon slices and extra blackberries and serve.