Combine the blueberries, water, granulated sugar, and salt in a medium saucepan. Bring to a gentle simmer over medium heat.

Cook for 7-10 minutes, stirring occasionally, until the blueberries have burst and softened, and the mixture has slightly thickened.

Remove the saucepan from the heat. Stir in the freshly squeezed lemon juice and finely grated lemon zest.

Carefully transfer the hot blueberry mixture to a blender. Blend on high until completely smooth. (Alternatively, use an immersion blender directly in the saucepan.)

Pour the blended mixture through a fine-mesh sieve into a clean bowl or container, pressing on the solids with a spoon to extract all the liquid. Discard the pulp. This step ensures a silky, smooth texture for the soup.

Cover the bowl and refrigerate the blueberry lemon soup for at least 2 hours, or until thoroughly chilled. The longer it chills, the better the flavors will meld and the thicker it might become.

Once chilled, give the soup a quick stir. Pour approximately 2-3 ounces (about 1/4 to 1/3 cup) into each shot glass or small serving cup.

Garnish each shot with a fresh mint sprig, a thin lemon slice, and a few whole blueberries, if desired. Serve immediately as a refreshing "shot" soup.