1. In a pot, combine the blueberries with 1 cup of the water. Crush the berries with a potato masher and bring to a simmer. Simmer over low heat for 15 minutes. Strain the juice into a heatproof measuring cup, pressing hard on the solids. Discard the solids.
2. Rinse out the pot. Add the sugar, lemon zest and the remaining 3 cups of water and bring to a boil, stirring to dissolve the sugar. Boil the syrup over moderate heat until it registers 225° on a candy thermometer, about 20 minutes. Add the blueberry juice and lemon juice and boil over high heat for 1 minute. Let the syrup cool, then discard the lemon zest. Pour the syrup into just-cleaned bottles. Seal and refrigerate for up to 6 months.