Preheat oven to 325°F (160°C). Prepare a 9x5-inch loaf pan, smearing the inside with butter. In a medium bowl, vigorously whisk together the flour, baking powder and salt.

Use an electric mixer (you can mix by hand but will get better results if you use an electric mixer) to beat together the butter, ricotta, and sugar, on high speed, for 3 minutes, until pale and fluffy. Add the eggs one at a time, mixing on medium speed for one minute after each addition. Mix in the lemon zest and vanilla extract. Don't worry if the batter looks a little curdled at this point. It isn't curdled, it just looks that way.

Remove the bowl from the mixer and stir in the dry ingredients in 3 or 4 additions, until just incorporated. Do not over mix. Stir in the blueberries.

Pour the batter into the prepared loaf pan, smoothing out the top with a rubber spatula. Place on a baking sheet on the middle oven rack of the oven. (This will help moderate the temperature at the bottom of the pan.) Bake for 65 to 75 minutes, or until a toothpick comes out clean. For the last 10 minutes of baking you may want to put a sheet of aluminum foil over the top of the pan to keep it from browning further.

Remove from oven and let cool for 15 minutes in the pan. Then run a dull knife around the edge of the cake to make sure it is separated from the pan. Gently remove the cake from the pan and let cool completely on a wire rack. Let cook completely before slicing.