Add gin, elderflower liqueur, blood orange juice, and egg white to a cocktail shaker, along with ice.
Shake vigorously until chilled, about 30 seconds.
Strain into a coupe glass and garnish with a slice of blood orange. Serve immediately.
Regular orange juice, freshly squeezed, can be substituted for blood orange juice.
About Egg Whites: Use a pasteurized egg white product instead of raw egg white to avoid contamination. If desired, omit egg white and top the cocktail with club soda instead.
If you prefer a classic gin fizz, here's an easy recipe: Add 2 oz. gin, 1 oz. lemon juice, and 3/4 oz. simple syrup to cocktail shaker with ice. Shake with one egg white, strain into glass, and top with club soda.