The following post appeared on my own blog a number of years ago but I wanted to share it here as well…
In all of my readings, I have found no one and nowhere that point to sangue di Giove(blood of Jupiter [Zeus]) as a philologically tenable origin of the grape name, Sangiovese.
This is what philologists call a folkloric etymology, most likely due to the quasi-homonymic (and Romantic) rapport between the enonym and purported etymon (the literal sense of a word according to its origin).
While divine blood plays a central role in Christian myth and liturgy (another element that most likely contributes to the folkloric etymology), it is not found in the Roman or Italian cults that honored Jupiter during the vinaliae (wine festivals) of late antiquity. (For the record, blood does play a role in the myth of Zeus, when the “blood from the birth of Zeus begins to boil up” in a “sacred cave of bees… said to be found on Crete.” See this profile of Rhea, who gave birth to the deity.)
Most scholars believe the most plausible etymon to be sangiovannina, a term which denotes an early ripening grape in the dialect of Sarzana, a township that lies on the border of Liguria and Tuscany in northwestern Tuscany. (Hohnerleien-Buchinger, 1996, cited in Vitigni d’Italia, eds. A. Scienza et alia).
It’s also possible that Sangiovese comes from sangiovannese, an ethnonym denoting an inhabitant of San Giovanni Valdarno, a town in the province of Arezzo. (I actually think this is the most likely answer to the conundrum; here’s a link to the Sangiovannese soccer team website.)