In a medium saucepan over medium heat, add the blueberries, water, and lemon juice.
Stir, and bring to a boil. Turn down the heat, and let simmer for 10 minutes.
Add the syrup to the saucepan, and let simmer for an additional 5 minutes.
In a small bowl, mix the cornstarch with water using equal parts cornstarch=water.
Slowing stir the cornstarch mixture into the blueberry sauce for about 5 more minutes. Stir over heat for your desired consistency. It will thicken very quickly.
Remove from heat, and it's ready to go!