Line a baking sheet with parchment paper.Stir the yogurt and vanilla in a bowl. Add 1/4 cup to 1/2 cup cold water to thin the yogurt out a little. Spread onto the prepared baking sheet in a thin layer. Add dollops of the jam, gently swirling into the yogurt. Scatter the blackberries on top. Drizzle over the white chocolate, then sprinkle on the brown rice crisp cereal. Freeze until very firm, at least 3 hours. Beak into pieces. Keep in the freezer.