Combine mashed blueberries and water. Squeeze 1/2 lemon, add peel and juice. Cook for about 15 minutes. Add sugar and stir well. Boil until sugar is dissolved, about one minutes. Remove from heat, add Certo. Stir and skim foam. Pour into hot sterilized jars, close. Let cool and freeze or seal with melted wax and store in cool dark dry place. Yield: 2 quarts