Peel and chop the tomatillos. Combine blackberries & tomatillos in a blender or food processor and puree until smooth (should be about 2 1/2 cups of fruit).

Mix 2/12 cups sugar and the pureed fruit in a saucepan. Add the lemon juice and stir.

Allow it to come to a full rolling boil for one minute, then decrease the heat and continue to cook for an additional 15 minutes. Skim any foam off the top, and pour the preserves into a jar.

Shake tequila, preserves, lime juice well over ice. Salt the rim of two glasses and pour cocktail into ice filled glasses. Top with a bit of club soda and a few blackberries.