The blood orange is a variety of orange (Citrus sinensis) with crimson, blood-colored flesh. The fruit ranges from small- to medium-sized; its skin is usually pitted but can be smooth. Blood oranges owe their distinctive appearance to a pigment called anthocyanin not typically found in citrus, but common in other red fruits and flowers. Not only is the inside of the orange darkly pigmented, but depending on the variety, the outside may feature dark washes of red. The exact reasons for the unusual inside color are unknown. However, it seems light, temperature and variety are important contributors. Citrus fruits have been cultivated in Italy since ancient times. In Sicily, the "blood" orange gained its nickname because its ruby flesh contains the red pigment anthocyanin, a strong antioxidant. Today, blood oranges are considered the hallmark of Sicilian fruit. Three blood orange varieties, Tarocco, Moro and Sanguinello, can be traced to the hilly areas and plains surrounding the Mt. Etna volcano in Sicily. Temperatures in this region range from intense daylight to frigid temperatures at night. It also has other variables, including modest rainfall and rich soil. The combination of these factors results in a Sicilian Red orange that has incomparable sweetness and distinguishable color. Sicily combines the juiciness of the Florida oranges with the appearance of the California counterpart which contains 20 percent. The fruit is easy to peel, and most varieties are seedless. The Sicilian Red orange is connected to its natural Mediterranean environment, resulting in fruit with superb flavor that cannot be replicated. These factors position Sicily as the largest producer in the world. It is also why the European Union recognizes the Eastern Sicily area as a Protected Geographical indication, Arancia Rossa di Sicilia, for cultivating Tarocco, Moro and Sanguinello varieties. http://tinyurl.com/2rsqje

Blood oranges are oranges of a different variety, their inside sections are slightly more red than your usual orange, and the skin pigment is darker. They contain anthocyanins (the same pigment in red grapes) which give them their colour. They are believed to be of Sicilian origin from fairly recent times... http://www.citrusvariety.ucr.edu/citrus/sweet_oranges.html