In a blender, combine fresh blackberries, granulated sugar, and lemon juice. Pulse until smooth, approximately 1-2 minutes. Strain the puree to remove seeds.

Sprinkle gelatin powder over cold water in a small bowl. Let sit for about 5 minutes until bloomed.

Microwave the bloomed gelatin for 10-15 seconds until fully dissolved. Allow to cool slightly.

Mix the warm gelatin into the blackberry puree, stirring until well combined.

In a mixing bowl, beat heavy whipping cream with vanilla extract and salt until stiff peaks form, about 3-5 minutes.

Gently fold the blackberry mixture into the whipped cream using a spatula in thirds.

Spoon mousse into serving dishes. Cover and refrigerate for at least 1 hour to firm up.

Garnish with fresh blackberries and whipped cream before serving.