Cut a wedge of lime and wipe it around the edge of your glass – either a margarita glass or whatever you prefer. Dip the glass into a plate filled with sea salt and set aside.
Put the blueberries (defrost them if frozen), agave nectar, basil leaves and lime juice in a cocktail shaker or glass and muddle them with a muddling spoon or the end of a rolling pin. Strain the blueberry pulp through a small sieve and press down with the back of a spoon to extract as much juice as possible from the berries.
Put the tequila, triple sec and blueberry juice in cocktail shaker half filled with ice and shake for 30 seconds.
Strain the cocktail into the prepared glass and add some cracked ice if you like. Garnish with a lime wedge and a small basil leaf.