Using a vegetable peeler, peel a (2-inch) strip of skin from the blood orange directly over a cocktail beaker to catch any oils; allow the peel to fall into the beaker.

Fill the beaker with ice.

Add the Campari and sweet vermouth and stir until very chilled, for at least 30 seconds.

Using a Julep strainer, strain the cocktail into an ice-filled rocks glass.

Top with club soda, to taste.

Peel another strip of skin from the blood orange and twist.

Rub the peel along the rim of the cup and drop into the cocktail; serve immediately.