Alan - what do you mean by “vineyard blends”? As others have pointed out, some vineyards are planted with a mix of grapes. Others are planted with these grapes over here, those grapes over there, etc.
Also, you have to realize that CdP isn’t somehow unique in that they blend grapes.
Obviously, blending the grapes is not the same thing as growing and harvesting them together. CdP and Bordeaux are both usually blends. Originally they were field blends, as were the wines in many, if not most places in Europe. In places like Piedmont, Rioja, etc., they usually had a favored grape but always had a little something else planted because you could never count on the weather in any given year. When the first immigrants planted CA vineyards, they felt the same way.
In the 1960s, people like Mondavi taught Americans to buy wine based on a specific grape variety. That’s become a new-world thing to some extent, although it’s showing up in Europe too. In any event, the result is that “field blends” are unusual in the US today. But if you look at a wine like Phelps Insignia, it’s a blend. And since the rules in CA say you only need 75 percent of a grape to call the wine by that grape, many wines are blends in exactly the way CdP is.
Another thing that happened in the US is that some people tried to copy the French models and consequently won’t blend “Bordeaux” varieties with “Rhone” varieties, although that never made any sense to me. It’s not as if those grapes somehow belong in either place and don’t work with the grapes from somewhere else.
In places like Priorat, there is no real tradition before 1990 so you can’t say anything about that region with certainty. The first five people who basically created the region found old Garnacha and Carinena growing. So they planted Cab, Merlot, Tempranillo and Syrah, because who ever heard of world class wines being made from Garnacha or Carinena? Since then of course, CdP achieved a status almost as lofty as Bordeaux and in Priorat today, after lots of experimentation, you find wine that is 100% Garnacha, 100% Carinena, and any number of other blends of various grapes.
In Rioja, they always blended but because the main houses bought from various growers, the wines were not “vineyard” blends. However, there are producers who have their own vineyards and who grow a mix of grapes in those vineyards. And I think that’s the way it was in most of the world.
Today in the US and elsewhere too, grape growing is kind of a monoculture, which tradition would have told us is just bad farming. We select the same clones and plant an entire vineyard with one clone of rootstock grafted with a single clone for the scion. That’s just not the way people did things in the past, in any country. And as to your example of CdP, it’s increasingly going the other way. You’re finding single-vineyard wines, 100% Grenache, etc. All of that is of course the antithesis of CdP as you’re imagining it.