In a medium sauce pan combine blueberries, ½ cup sugar and lemon juice. Mash the blueberries with a spoon or potato masher to release some juices.
Cook over medium heat, stirring occasionally (and mashing more blueberries) until mixture is thick like jam and reduces to about 1 ¼ - 1 ½ cups. Remove from heat and let cool.
Cake
Preheat oven to 350°F. Grease an 11 x 17 inch or 13 x 18 pan with a little butter or shortening. Line with parchment paper and grease the top of the parchment paper.
In a medium bowl, whisk together flour baking powder and salt.
In a large mixing bowl, whisk together eggs on medium-high speed and gradually add ¾ cup sugar and lemon extract. (Use a stand mixer with whisk attachment if available).
Continue whipping until very thick and fluffy, about 4-6 minutes.
Fold flour mixture gently into the egg mixture.
Spread batter into prepared pan and bake until center springs back when touched, 12 to 17 minutes (the size of your pan will alter the cooking time so start watching at 12 minutes).
Remove cake from oven and sift some powdered sugar lightly over the top. Place a clean dishtowel on top. Place a large cutting board, flat tray, etc. on top of the towel and invert the cake. Remove the pan and gently remove the parchment paper.
Roll the cake and the towel from the short end. Place on a cooling rack seam side down and let cool about 15 minutes.
Final assembly
Unroll cake. Spread blueberry jam over the cake leaving a ½ inch border. Reserve some jam for decorating the top if desired.
Reroll the cake tightly and place on a serving plate. Let cake cool completely. Dust with powdered sugar and decorate with remaining jam and lemon slices if desired.