BEAT SUGAR AND EGG YOLKS together until pale yellow and very thick. Slowly add milk, beating gently to avoid a build-up of foam. Add lemon peel and stir in salt. Transfer mixture to the top of a double boiler with an inch of boiling water in the bottom half. Regulate heat so water remains at a low boil, and stir continuously 8 minutes. Custard will thicken to coat the spoon, and surface foam will disappear. Immediately remove top of double boiler and set it in a large bowl of cold water. Stir 2 minutes to cool custard somewhat. Then transfer it to a bowl, cover, and chill thoroughly. Also chill pureed blueberries and heavy cream. When ready to proceed, whip cream into soft peaks. Remove lemon peel from custard and fold in blueberries and whipped cream. Transfer mixture to ice cream machine and freeze according to machine's directions. Makes 1 Quart