The way it was explained to me by an Italian friend/neighbor/farmer, Chuck Giovannetti, with a mixed black vineyard in the RRV:
Vineyard was acquired in payment for back wages owed by the owner when he went bankrupt in the early 1900’s. Miro (Chuck’s grandfather) moved to the property and lived in a tent after the '06 Quake. The vineyard was originally planted in Carignane in the late 1800’s but was grafted over the mixed blacks in 1910.
The reason for the “Dago Blend” as he refers to it is that the different varieties ensure there is a good wine to be made year in and year out regardless of the weather. Each variety brings something different to the blend, Fruit, acid, tannin etc…