Blackberries have enough pectin in them to gel without adding any extra pectin to the recipe. With that said, if you'd like to keep these preserves less 'jelly-ish', don't allow the mixture to come to a rolling boil. 

Place a spoon in the freezer. Peel and chop the tomatillos. Combine blackberries & tomatillos in a blender or food processor and puree until smooth (should be about 2.5 cups of fruit). 

Mix 2.5 cups sugar (equal parts sugar:fruit), and the pureed fruit in a saucepan. Add the lemon juice, and stir the mixture; 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.  

Remove the spoon from the freezer, and dip into the preserves. Hold at least 12 inches above the saucepan and turn the spoon to pour the preserves back into the pan. If the preserves sheet together, and hang off the side of the spoon, the jelly is ready. See this trick in more detail here. 

Skim any foam off the top, and pour the preserves into 2-3 8 oz. jelly jars. Since this recipe has not been scientifically tested, I suggest keeping it refrigerated for up to three weeks.