Not likely! Most every Cab by every winemaker is a blend to some degree, just not to the same higher percentages as in Opus or other meritage blends.
Typically a few a varying percent of Merlot, Cabernet Franc, Petite Verdot and Malbec are blended in. They are not required to put that on the label if up to 25% of the total volume.
There are exceptions like Robert Mondavi Stags Leap District Cabernet Sauvignon, or Caymus Special Select (and other high-end, low volume producers) that reflect 100% varietals. But they are few and far between and usually out of the price range of most buyers would ever get the chance to experience! And if they are 100% they tend to put it on labels because it is a bragging right.