Port has two primary varieties - Vintage Port that is bottled soon after fermentation and carries a year designation, as suggested by the name and Tawny Port, that is slowly aged in wood and blended over a period of years. The distilled spirit aguardiente is added to all Ports, and is controlled by the government, not the winemaker. Thus the quality variation is due to the nonfortified or natural components of the port and the processing decisions. Rancio is the Spanish term for the flavors arising from the carmelization and oxidation of these wines. Dom Perignon was celebrated in his day for expert blending, especially of a white wine made from Pinot Noir.
Varietal labels are a fairly recent trend.
Frank Schoonmaker advised Almaden to do this in the 1930s and that started the ball rolling. To some, it has destroyed some traditional blending practices, e.g. in Italy.