Blood oranges are a citrus fruit that has dark crimson flesh and a darker red/orange rind.  They contain a red pigment, common among flowers, that is a potent antioxidant.  they also have the most vitamin C of any orange variety on the planet. 

Blood oranges come in three types: tarocco, sanguinello, and moro.  The tarocco, which is the most popular table orange in Italy where it originated, is the sweetest and most flavorful of the three.  The sanguinello, discovered in Spain in 1929, has few seeds, is sweet and tender, and has flesh that is orange with red streaks rather than the darker moro.  The moro has deep purple flesh and a red/orange rind.  It has a hint of raspberry to it's tang and is the newest and most striking variety to date.