Place the frozen blueberries and strawberries into a blender or food processor, and pulse until broken down into small pieces.

Add the mixture to a saucepan over medium heat, and stir in the sugar substitute.

Simmer until the mixture comes to a boil. Boil for 5 minutes, and remove from the heat.

Pour the mixture carefully through a sieve and into a measuring cup.

When all of the liquid has been drained from the berries, fill the measuring cup up the rest of the way to the 2 cup mark with water. Taste and add more sugar substitute if desired.

In a bowl, stir together the 1 cup of water and gelatin powder.

Pour the berry mixture into the gelatin mixture, and whisk to combine well.

Layer the mixture into a loaf pan, and place into the fridge for several hours until completely set.

Slice the Jello into cubes and serve.