Combine the tequila, Cointreau, blackberries and sage leaves in a blender. Pulse twice, then blend on high speed for 4-6 seconds, just until well blended.

Strain the mixture through a fine mesh sieve (stirring speeds up this process considerably!) into a mixing bowl, add the remaining ingredients, and stir. Pour into a flat bottom-ed pan or baking dish (13-by-9-by-2-inch works best) and cover with plastic wrap.

Place the pan in the freezer, then stir thoroughly with a fork (make sure you really scrape the sides!) every half hour for about 4 hours, until the consistency is light and fluffy, almost like sorbet.