Combine all ingredients in a medium-sized sauce pan and stir together. Bring the mixture to a boil, then lower to a simmer and cook, stirring occasionally, for about 10 minutes. The mixture will have thickened slightly and the blueberries will be very soft.
Carefully transfer to a blender or food processor and purée. Strain the mixture using a fine mesh sieve over a bowl, discard the solids.
Allow the blueberry mixture to cool slightly then pour into a glass or metal bowl, cover and place in the freezer.
Every hour to hour and a half, scrape the ice crystals that form with the tines of a fork. Continue to do so, every hour or so, until the granita is completely frozen, about 5 hours.
When ready to serve, use a fork to scrape the granita and scoop into a bowl or glass. If a smoother consistency is desired, place the granita in a food processor and blend until smooth.
Serve as is or with a brioche and dollop of whipped cream.