Combine the first 4 ingredients in an ice-filled pitcher. Pour into ice-filled rocks glasses. Top off each with a dash of bitters, if desired. Garnish, if desired.
Blood oranges differ from their thick-skinned cousins, navel oranges, because they are less acidic in taste and have a richer, berry-like flavor. When pressed, blood oranges yield a tangy-sweet juice that pairs well with bourbon.