Bring the blueberries, sugar, water, and salt to a simmer in a medium saucepan over medium heat. Stir occasionally and cook until the sugar is dissolved, about 2 to 4 minutes.

Transfer the mixture to a blender or mini-processor and puree until smooth. Strain the puree through a fine-mesh strainer, pressing on the solids to extract as much puree as possible (you should have about ½ cup). Spoon the sauce evenly into six 4-ounce ramekins, and refrigerate until chilled, about 20 minutes.

While the blueberry sauce is chilling, sprinkle the gelatin evenly over the water in a small bowl. Set aside to let the gelatin hydrate for 10 minutes. 

In a separate bowl, whisk the yogurt, heavy cream, lemon zest, lemon juice, vanilla, and salt together until smooth. Set aside.

In a large heatproof bowl, whisk the egg whites, cream of tartar, and sugar together. Set the bowl over a pot or saucepan of barely simmering water, making sure the water does not touch the bottom of the bowl.  Heat the mixture, whisking constantly, until it has tripled in size and registers about 160 degrees on an instant-read thermometer, 5 to 10 minutes.

Once removed from the heat, whisk in the hydrated gelatin until dissolved.  Whip the warm mixture with an electric mixer on medium-high speed until it forms stiff, shiny peaks, 4 to 6 minutes.  Add the yogurt mixture and continue to whip until just combined, about 1 minute longer.

Divide the mousse evenly among the chilled ramekins with the blueberry sauce, cover tightly with plastic wrap, and refrigerate until chilled and set for at least 6 to 8 hours. Serve chilled. If desired, garnish each serving with several berries and a dusting of lemon zest.