Add 2 cups of blueberries, sugar, and water to saucepan. Heat over medium heat until mixture is simmering and blueberries start to burst. Stir in dissolved cornstarch and increase heat to medium high. Bring mixture to a boil and let boil for two minutes. Remove from heat and stir in remaining cup of blueberries and vanilla extract.
In medium sized bowl, mix cream cheese and powdered sugar with hand mixer until well combined. Stir in lemon juice and lemon zest.
Heat nonstick griddle to medium heat.
In a medium sized bowl, vigorously whisk together eggs, milk, sugar, cinnamon, vanilla, and salt.
Spread cream cheese mixture over 6 pieces of brioche bread. Top with fresh blueberries and press gently into cream cheese. Top with another piece of brioche bread and press together gently.
Add butter to preheated griddle. Dip assembled French toast into egg mixture letting excess drip back into bowl. Place on hot griddle and cook until golden brown on both sides and cream cheese is warm and melting. Serve with blueberry sauce over top.