Mix the salt and sugar together on a small plate. Run a lime wedge around the rims of two cocktail glasses, then dip the rims into the salt and sugar mix. Set rimmed glasses aside until ready to use.
Muddle blackberries in the bottom of a large cocktail shaker. Fill the shaker with ice, then add lime juice, orange juice, tequila, and agave. Secure the lid onto the shaker, and shake vigorously for about 30 seconds.
Fill the rimmed glasses with ice. Remove the lid from the shaker, and strain the shaker contents into the two glasses. Garnish with a lime wheel and blackberry!