Cara Cara oranges are, apparently, a recent hybrid of navel orange varieties. According to Produce Hunter the likely suspects are the familiar Navel --known as the Washington variety -- and the new to me Brazil Bahia Navel. I'm not sure all of this washes - other histories describe the Washington as a renamed Bahia. If you have the straight dope, go ahead an send it my way.
Most descriptions of the Cara Cara flavor relate it to Moro Blood Oranges. At first blush though, I tasted sweet grapefruit. These photos don't really convey just how pink the flesh of the orange is.
I have a confession to make. We get our Cara Cara oranges from the Hamada Farms booth at the Grand Lake Famer's Market. I noticed them just as I was getting over my unfortunate malreaction to Satsumas, and found no mention of them on any of the web sites describing seasonal citrus. I assumed that they were an incidental rathern than deliberate hybrid of navel orange and grapefruit.
I shared this assumption as fact with several folks, to whom I will now openly apologize. In a fitting bit of irony, my presumption was done in when my wife pointed out that she had seen them in a post to I blame the Patriarchy.