In a blender or food processor, purée the blackberries until smooth. Strain through a fine mesh sieve to remove seeds and collect about 1 cup of berry purée. Discard seeds.

In a small bowl, sprinkle gelatin over cold water and let bloom for a few minutes until softened. Then stir into saucepan.

In a medium saucepan, heat the blackberry purée, sugar, and lemon or lime juice over medium heat, stirring often, until small bubbles form at the edges and sugar dissolves (~5–7 minutes). Add the softened gelatin and stir until completely dissolved (~1 min). Remove from heat and let cool for about 20 min to room temperature.

While the blackberry mixture cools, whip the chilled heavy cream with vanilla extract until soft peaks form.

Gently fold half of the cooled blackberry gelatin mixture into the whipped cream to lighten it, then fold in the remainder, taking care to preserve the mousse’s airiness.

Spoon the mousse into individual serving glasses or ramekins. Chill in the refrigerator for at least 2–3 hours or until set (overnight works well too).