In a small bowl, combine lemon juice and cup cold water
Sprinkle gelatin on top and let sit until softened, 2 minutes.
Fill a medium bowl with ice water
In a small saucepan, combine gelatin mixture, sugar, and pinch of salt over medium heat and stir until gelatin and sugar dissolve, about 4 minutes.
Pour gelatin mixture back into small bowl and set in ice bath. Stir until mixture reaches room temperature, about 1 minute.
In a large bowl, whisk cream until soft peaks form.
Pour cooled gelatin mixture into whipped cream and whisk until soft peaks return.
Divide evenly among four 8-ounce dishes or cups and place in freezer until mousse springs back lightly when pressed, 10 minutes.
Serve topped with blackberries.
Serving Size Serves 4