Set 4 (4 oz.) ramekins in a large baking pan. Bring a kettle of water to boil. Heat the oven to 325°F.
Combine the egg yolks, lemon zest, and 1/4 cup of the sugar in a medium bowl. Combine the milk, cream, salt, and remaining sugar in a small saucepan and bring to a simmer. Slowly and gradually whisk the warm cream mixture into the egg yolk mixture. Divide among the ramekins.
Set the baking pan in the oven. Pour boiling water into the baking pan until it reaches halfway up the sides of the ramekins. Cover the pan with foil.
Bake the custards until they are set around the edges but still jiggle a bit in the center, about 25 to 30 minutes. Remove the pan from the oven and let cool to room temperature, then transfer the ramekins to the fridge to chill for at least 4 hours. To serve, top with whipped cream and blueberries, if desired.
Note: Total time includes chilling time.