1. Heat the cream over very low heat in a small non-stick saucepan.
2. Add the white chocolate pieces, a little at a time. Make sure the mixture is smooth and homogeneous.
3. Remove the saucepan from the heat, let it cool, then place the mixture in the refrigerator for at least 2 hours.
4. In a saucepan, add the blackberries and sugar. Simmer over low heat for 10 minutes, covered.
5. Remove the mousse from the refrigerator and whip it with an electric mixer.
6. Divide the mousse into small cups or individual bowls.
7. Add the blackberry compote and serve chilled.