Varietal wines are simply wines that are named for the grape principal grape variety used to make it. Some varietal wines are made 100% from the grape named on the label. Varietal wines made in the US must be made from at least 75% of the named grapes. Regulations in other places range from 100% on down to nothing.
There is nothing wrong with blending grape varieties. Many of the world’s greatest wine are made from blends. While some grapes such as Chardonnay and Pinot Noir work very well on their own, they both blend well and others such as Cabernet Sauvignon and Grenache tend to be better blended. There are of course always exceptions.
Varietal names became popular in the US in the 1970’s and 1980’s. The consumption of bottled wines was exploding bringing many new people to the pleasure of a good glass of wine. Varietal names made things easier. Rather than having to know a huge number of wines named for their place of origin, one could just ask for a Chardonnay or a Cabernet Sauvignon. These two varietal wines became so ubiquitous that some wines list had what they called “ABC” wines, anything but Chardonnay and Cabernet Sauvignon.
Predictably, the fashion for varietal wines faded. In part this was a reaction to the exploitation of the big names, like Chardonnay, by less quality oriented producers. Also wine drinkers and some producers were becoming more sophisticated, discovering the multitude of grape varieties as well as the benefits of a good blend. Things seem to be a good balance now with a large number of quality varietal wines being available in a wide range of prices as well as producers who put out fine blends. Enjoy.