You'll need (makes 3 glasses):

2 cups blackberries
2/3 cup sugar
1 cup water
1 lime
3 fl oz Cointreau
3 fl oz Grand Marnier
4 fl oz tequila
4 fl oz sugar syrup
3 tbsp sea salt

Here's how:

Let the blackberries, sugar and water simmer together in a saucepan for 10 minutes. Strain the contents of the pan through a sieve into a bowl and push the blackberries through the sieve. Leave the syrup to cool.
Pour the cooled blackberry syrup into a glass pitcher and add the juice from both halves of a lime, the Cointreau, the Grand Marnier and the tequila, and mix everything together.
To serve in the traditional style, dip the rim of the glasses in the sugar syrup and then in the sea salt. Pour in the Margarita and, last but not least, decorate the glass with a slice of lime and pop in a straw.