Strain through fine sieve. Pour into baking pan (or transfer to ice cream maker and process according to manufacturers' directions). Freeze uncovered until edges are solid but centre is soft. Transfer to processor and blend until smooth again.
Return to pan and freeze until edges are solid.
Transfer to processor and blend until smooth again.
Fold in remaining 2 cups of blueberries.
Pour into plastic mold and freeze overnight. Let soften slightly to serve.