A few weeks ago, I wrote about red blends as a growing wine category. I personally find blends often make more complete and complex wines than varietal wines. Aroma, flavor or texture characteristics that may be missing from a single variety can be filled by another grape. It is amazing to see that as little as 1% of a variety blended in truly can completely transform a wine. Blending is also a useful tool for a winemaker to create as good of a wine as he or she can each year. Weather conditions in back-to-back vintages can treat cultivars quite differently, so by playing with the cépage (percentage of each variety of grapes in the composition of a blended wine) can adjust the characteristics of the finished wine.
I find that many wineries take two different approaches to blends. The first is to make the blend their grand vin - top wine. The best lots are used to build the best wine possible. Often with this approach, the same cultivars are used to create the blend. The cépage may change - or it may stay exactly the same - but the building blocks generally are the same each year. Perhaps certain vineyards, or blocks, are selected for the vintage characteristics each provide. What is "leftover" after the blend is finalized can then be sold as varietal wine or different blend (or off in bulk anonymously).
The second approach is to create a blend after all the varietal, single-vineyard, and premium blends have been finalized. This approach can be perfect for creating great value wines - if the winemaker still takes care in making sure the blend works. All too often, simply throwing all the "leftover" wine together doesn't produce a high-quality wine. Today's wines are examples of both these approaches, and both are beautiful wines in their own right.