Combine 1 cup of the sugar, 1/3 cup of the sage and 1 cup water in a medium saucepan. Bring to a simmer, allowing the sugar to dissolve. Let the mixture cool for about 30 minutes, then strain.
Combine the salt and the remaining 2 tablespoons sugar and 1 tablespoon sage in a shallow dish. Lightly dampen the rim of a margarita glass with some of the key lime juice. Dip the rim in the salt mixture to coat. Thread 2 of the blackberries on a small skewer.
Muddle the remaining 8 blackberries n a cocktail shaker. Add the tequila, orange liqueur, key lime juice and 2 ounces of the sage simple syrup. Fill with ice and shake until the shaker is cold, about 10 seconds. Pour the mixture into the prepared glass and garnish with the skewered blackberries.
Reserve the remaining simple syrup for another use. It will keep in the refrigerator, covered, for up to 1 month.