Remove blueberries from the freezer and allow them to thaw on paper towels. Note: They don't need to be completely thawed.
Meanwhile, in a shallow bowl, beat the eggs, milk, sugar, vanilla, and salt with a whisk until well blended. Place both halves of one croissant into the egg mixture and soak them without oversaturating. Allow excess batter to drip back into the bowl and then transfer the halves to a platter. Spread 1/4 cup of blueberries onto one half of the croissant. Top with the other half and press down firmly. Repeat with the remaining croissants.
Preheat oven to 200 degrees.
Heat 1 tablespoon of butter and the canola oil in a large skillet over medium heat. When the butter starts to bubble, add the filled croissants. Cook until browned, about 4-6 minutes. Turn them over and cook on the other side until browned, or 4-6 minutes.
Transfer the cooked croissants to a baking sheet. Place in 200 degree oven to keep warm while you make the blueberry sauce.
In the same skillet, melt the remaining 2 tablespoons of butter over medium heat. Add the brown sugar and let the mixture foam. Then add in the remaining blueberries and syrup. Stir well and let simmer 4 minutes. Place the croissants on individual plates and quickly ladle the sauce over each croissant. Serve immediately.