Blending Wine

Blending of wines, both in the red and white categories, is very common in many places around the world.
However when sold under a regional label eg. Bordeaux, it is almost impossible to know what grapes make up the wine in the bottle.
Cabernet Sauvignon, Cabernet Franc, Merlot, Malbec and Petit Verdot are the permitted red grapes grown in that area and wines are usually a blend of some of those. Cabernet Sauvignon dominates wines from the left bank of the Gironde and Merlot is the major component in wines from the left bank.
Whites from that area are usually a blend of Semillon and Sauvignon Blanc although there are a number of permitted lessor known varieties.
In contrast Red Burgundy is always 100% Pinot Noir. Of course Gamay is used for Beaujolais but that wine is marketed under its Burgundy regional name.
In my area of special interest, Rioja in Spain, a typical red wine blend will consist of approximately 60% Tempranillo and up to 20% Garnacha, with much smaller proportions of Mazuelo and Graciano.
Each grape adds a unique component to the wine with Tempranillo contributing the main flavors and aging potential, Garnacha adding body and alcohol, Mazuelo adding seasoning flavors and Graciano adding additional aromas. In some special cases the use of Cabernet Sauvignon is allowed in the blend.
Australia (and many other New World countries) generally markets wine according to its variety.

The law here says that a wine must contain at least 85% of a variety to be called a single varietal. The origin of the remaining 15% does not require identification.
Wines sold as blends must have the various component wines listed in descending order.
A typical Australian blend not found in many other places is Cabernet Sauvignon/Shiraz.
I have never considered blending here but this year decided to see what could be achieved with the higher quality wine produced this vintage.
I left the Pinot Noir as a straight varietal and looked only at a Tempranillo/Cabernet blend starting with 15% then 20% and 30% of the latter.
The tasting panel consisted of myself and the co-driver.
We both agreed that the 20% Cabernet addition made an interesting wine discernibly different from both the Tempranillo and Cabernet themselves. The lower end addition did not alter the base wine markedly and higher end addition really did nothing for the base wine.
So we shall blend off half the Tempranillo that way and will have four red wines to offer our 'customers' from the 2014 vintage.
The Semillon has been bottled and is in the 'market place' for evaluation.