The blood orange is a variety of orange (Citrus sinensis) with crimson, blood-colored flesh. The fruit is smaller than an average orange; its skin is usually pitted, but can be smooth. The distinctive dark flesh color is due to the presence of anthocyanin, a pigment common to many flowers and fruit, but uncommon in citrus fruits. Sometimes there is dark coloring on the exterior of the rind as well, depending on the variety of blood orange. The degree of coloration depends on light, temperature and variety.
The blood orange is often described as a hybrid between the pomelo and the tangerine, but it is actually just a mutation of a sweet orange.
try one if you haven't yet.
all info from google....and first photo.