Preheat the oven to 350 degrees F and position a rack in the middle. Butter a 9-by-2-inch round cake pan or a 9-inch springform pan and dust with sugar, tilting the pan to coat and discarding any excess.

In a large bowl, with an electric mixer on medium speed, beat the mascarpone, cream cheese, 3/4 cup sugar, 1 teaspoon of the lemon zest, lemon juice, and vanilla until smooth, about 2 minutes, scraping down the sides of the bowl once. Add the eggs, one at a time, beating well after each addition. Turn the mixer to low and add the flour, scraping the bowl to make sure it is incorporated.

Pour the batter into the pan. Bake until the top of the cake is a deep golden brown and a toothpick inserted in the center comes out clean, 35 to 40 minutes. Let the cake cool in its pan on a rack for 15 to 20 minutes. Run a knife around the sides of the cheesecake. If using a cake pan, invert a serving plate over the pan and reinvert the cheesecake on the plate. If using a springform pan, remove the sides and set the bottom on a serving plate. Let cool completely.

In a medium bowl, mix the blueberries with the 2 tablespoons sugar and the remaining 1 teaspoon lemon zest. If using fresh berries, crush a few with the back of a spoon to encourage juices to form. (Taste the fruit; if it's too tart, add another teaspoon of sugar.)

When ready to serve, dust the cheesecake with confectioners' sugar, if using, and spoon the blueberries over the cheesecake. Cut and serve with a dollop of whipped cream or creme fraiche, if you like.