
Blueberries are small round berries with deep blue-purple skin and pale flesh, mild and only lightly tart.
Muddled, they give sweetness and a soft tartness, and the drink's color depends on the acid. The pigment is anthocyanin, an acid indicator: with citrus it shifts from blue-purple toward pink-red. So a blueberry sour ends up several shades redder than the berries. They suit muddled drinks such as caipirinhas and fruit daiquiris, and blend well into shakes.
Frozen blueberries
The closest swap, measure for measure. Thawed berries are soft and bleed their color, so muddle them rather than using them whole.
Bilberries
The wild European cousin gives an even darker color.
Work in the same amount but are half as sweet. Add 2:1 syrup half a barspoon at a time and taste.
Replaces sugar syrup if the drink calls for it. The flavor and color come through, minus the crushed fruit. Monin's is close to a 2:1 syrup in sweetness.
Ten days to two weeks in the fridge, unwashed until you use them: water left on the skins starts the mold. Choose firm berries with the pale bloom still on them; the bloom is the fruit's own wax. A wrinkled berry has already lost juice.