Preheat oven to 325°F (160°C). Place six 6-ounce ramekins in a deep roasting pan and set aside.
In a medium saucepan over medium heat, combine the heavy cream, milk, and 1/4 cup sugar. Stir occasionally until the mixture is steaming and sugar dissolves completely, about 5 minutes. Do not let it boil.
In a separate bowl, whisk together the egg yolks, remaining 1/4 cup sugar, and salt until pale and smooth, about 1 minute.
Slowly pour 1/2 cup of the hot cream mixture into the egg yolks while whisking constantly to temper the eggs. Then pour the tempered egg mixture back into the saucepan with the remaining cream, whisking continuously.
Remove from heat and stir in the vanilla extract. Strain the custard through a fine-mesh sieve into a large spouted measuring cup or bowl to remove any cooked egg bits.
Divide the blueberries evenly among the ramekins. Pour the custard over the berries, filling each ramekin about 3/4 full.
Carefully pour boiling water into the roasting pan until it reaches halfway up the sides of the ramekins. Cover the entire pan loosely with aluminum foil.
Bake for 35-40 minutes, until the edges are set but the centers still jiggle slightly when gently shaken.
Remove the ramekins from the water bath using tongs and let cool on a wire rack for 30 minutes. Cover each with plastic wrap and refrigerate for at least 4 hours or overnight before serving.
