Cut the gelatin into 1-inch strips and submerge in cool water to soften.
Warm the ½ cup of blueberry puree until smooth and just bubbling. Wring on the gelatin to remove excess water and whisk the gelatin into the warm puree. Set aside to cool.
In a medium bowl whisk together the mascarpone cheese, sugar, salt, zest, lemon juice, and vanilla extract. Add the cooled blueberry mixture and whisk until smooth.
In a separate, clean bowl of a stand mixer, beat the heavy cream until it reaches stiff peaks. Add ½ of the blueberry mixture and fold to lighten the whipped cream then add the remaining cream and fold until fully incorporated.
Immediately begin assembling your cake (see recipe titled Blueberry Lemon Mousse Cake Assembly
Recipe by Her Modern Kitchen at https://www.hermodernkitchen.com/blueberry-lemon-mousse-cake-with-mirror-glaze/