And there are wines, and grapes, that are 100% or nothing. The two that lose far more than they gain by any blending at all are Pinot Noir and Riesling. Many would argue that they are, simply, the greatest soloists in the world of wine. Their greatness lies in their purity.
What other grapes usually get all the way to the bottle on their own? Chardonnay, most prominently. It's not that the taste of another variety would sully its particular purity; more that Chardonnay offers such a perfect package of flavour. Oak offers it another dimension, but it can work even better without it. Think of Chablis. As for adding a dollop of Sauvignon or Riesling: ridiculous.
Simple fruity flavours are found more often in white grapes than in red. Hence the familiarity of Sauvignon, of Pinot Gris, of Gewürztraminer and Muscat. Among reds there are fewer. Gamay (as in Beaujolais) is atypical. Red wine is (and usually should be) a more structured drink. When you ferment the skins with the juice you get away from simple (or not so simple) juice flavour. More depends on the judgement, and on the taste, of the winemaker, less simply on the grape.
The world's most famous blended wine - blended in the sense of combining two or more different grape varieties - is claret. The recipe is no secret. Cabernet Sauvignon plus Merlot plus Cabernet Franc is the Médoc version. The Cabernets give the punch, the smell of berries and herbs and the need for a certain age. Merlot rounds it all out, gives depth and softness and its own berry-like smell. The three grapes are close enough cousins to be confused: very far from chalk and cheese, but perfect complements. Merlot covers for Cabernet when it fails to get to perfect ripeness (Merlot ripens two weeks earlier). Cabernet covers for Merlot when it falls short on tannin, acidity and general grip.
Recipes like these have been worked on for centuries. In the past, when a better grape variety came along there was no hesitation in adding it to the mixture. Cabernet Sauvignon was added (and Malbec dropped) from the 18th century on. No one can imagine that happening today.
Even the noblest grapes, the untouchable Pinot Noir and Chardonnay, can be complementary. What is unthinkable in Burgundy is a perfect recipe in Champagne. Chardonnay gives the apple flavours, Pinot Noir the red fruit and the gravitas (and Pinot Meunier, not officially known in Burgundy, more red fruit).