Combine the water, lemon juice and sugar in a medium saucepan, stirring well. Add the blueberries and turn the heat on to medium to bring the mixture to a boil. Lower the heat and simmer the blueberries for 7-8 minutes until they start to burst. Combine the cornstarch with 1 1/2 tablespoons of water and stir it into the blueberry mixture. Raise the heat and bring to a boil. Once the mixture boils and thickens, remove the pan from the heat to cool.