Blue Sauce for Ice Cream

2 tablespoons butter
1 tablespoon flour
1 16-ounce package frozen blueberries
1/4 cup sugar, or to taste
1 tablespoon lemon juice
1 teaspoon lemon zest (optional)
Pinch salt
2 tablespoons water

Melt butter in a saucepan. Blend in all other ingredients. Cook over medium-high heat, stirring until berries disintegrate and thicken, about five minutes.

Makes about 2 cups.