Cook blueberries in water for 15 minutes to extract all juice.
Add sugar and lemon juice to blueberry liquid.
Return to heat and cook for 5 minutes.
Remove from heat and cool.
Divide mixture between 2 ice trays and put into freezer.
In 1 hour, stir with spoon.
Repeat at intervals until mixture begins to set.
When whole tray has partially frozen, whip cream very stiff and stir some into frozen mixture in each tray.
Return to freezer; stir occasionally while mousse is hardening to be sure cream does not separate.
Let freeze overnight.
Serve in sherbet or parfait glasses.