Bring milk to a boil in a saucepan; add rice and reduce heat to low. Simmer until rice is tender, about 20 minutes. Stir in sugar and salt. Remove from heat and allow rice to cool.
Beat heavy cream in a bowl until stiff peaks form. Lift your beater or whisk straight up: the whipped cream will form sharp peaks. Fold whipped cream into cooled rice.
Divide rice into 4 bowls and garnish with a few blackberries. Serve remaining blackberries on the side.
