Sprinkle the gelatin over the cold water in a small bowl and let it bloom for 5 minutes.
Combine the heavy cream, whole milk, and granulated sugar in a saucepan over medium heat. Stir until the sugar dissolves and the mixture is hot but not boiling.
Whisk the matcha powder into the warm cream mixture until fully smooth and no lumps remain.
Add the bloomed gelatin to the saucepan and stir until it melts completely into the liquid.
Remove the pan from heat and divide the mixture evenly among four small glasses or ramekins.
Chill the panna cotta in the refrigerator for at least 4 hours until fully set.
Top each serving with a few fresh blackberries just before bringing them to the table.