Put the blueberries in a pot over low to medium heat, do not add water, there will be enough liquid from the thawing of the berries. We like our sauce quite tart, if you like it sweeter, add more Splenda or sugar. Stir the Splenda into the berries, cook stirring until the berries are thawed and the Splenda dissolved; about 5 minutes.
Whisk together the cold water and the cornstarch until smooth, add mixture to the berries and simmer and stir until thickened, about 5 minutes. Try to keep most of the berries whole as much as possible. The recipe can be doubled or more.