Combine first 4 ingredients in top of a double boiler; bring water to a boil. Reduce heat to low and cook, stirring constantly, until butter melts.
Gradually stir about one-fourth of hot mixture into eggs; add to remaining hot mixture, stirring constantly. Cook in top of double boiler, stirring constantly, 20 minutes or until thickened. Chill 2 hours. Spoon 1 teaspoon filling into each tart shell. Garnish each tart with whipped cream and blueberries, if desired.