Mix together the lime zest, chilli powder and salt on a small plate. Cut the lime in half and run one half around the rim of a glass, then dunk this into the salt so it sticks to the rim. Juice both halves of the lime and pour this into the glass along with the blueberries and agave syrup. Pound with a muddler or the end of a rolling pin until the blueberries are thoroughly puréed. Add a handful of ice, then pour over the tequila and triple sec. Give everything a stir before serving.
Alternatively, to make a pitcher, multiply all the ingredients by eight, and put the blueberries, lime juice and agave into a blender. Whizz to a purée, then tip into a jug with the ice, tequila and triple sec, and stir. Serve in eight salt-rimmed glasses.
