Cleansing: Start by cleansing the blackberries under cold water. Combine blackberries with granulated sugar and lemon juice in a blender. Blend until smooth, about 30 seconds. Strain for a seedless texture if desired.

Gelatin Bloom: In a small bowl, sprinkle gelatin powder over cold water. Let sit for 5 minutes to bloom.

Heat Gelatin: Place bloomed gelatin in the microwave for 10-15 seconds to dissolve, stirring gently. Allow cooling slightly.

Combine: Stir the dissolved gelatin into the blackberry puree until evenly distributed.

Whip Cream: In a separate bowl, whip heavy cream with vanilla and salt until stiff peaks form, about 3-5 minutes.

Fold Together: Gently fold the blackberry mixture into the whipped cream, ensuring not to deflate the cream.

Chill: Spoon mousse into serving dishes, cover with plastic wrap, and refrigerate for at least 1 hour.

Garnish: Before serving, add garnishes like fresh blackberries or whipped cream as desired.