At any rate, if heavy body, seductive aroma, a muted acidity, and (in the case of American-style espresso blends) sweet-toned pungency are the goals, how does one attempt to achieve these goals in blending?
In general, roasters choose one more coffees that provide a base for a blend, additional highlight coffees that contribute brightness, energy and nuance, and (often but not always) bottom-note coffees that intensify sensations of body and weight.
Base Coffees. Coffees that provide the base for espresso blends should not be neutral, at least not in my view. They definitely should not be dull or bland. They should be agreeable, without distracting idiosyncrasy or overly aggressive acidity, round and sweet, and they should take a dark roast well. The classic base for espresso is Brazil coffees that have been dry-processed, or dried inside the fruit, a practice that promotes sweetness and body. Peru and some Mexico coffees (all of which are wet-processed) also make a gentle but lively base for espresso. Coffees from the Indonesian island of Sumatra are favorite base coffees among many West-Coast-style American blenders. Like Brazils, Sumatras are low-keyed, rather full-bodied coffees, but brought to a dark roast they tend to be bittersweet and pungent, qualities suited to the dark-roasted, sharp, milk-mastering blends favored by caffe-latte-happy West-Coasters.
Highlight Coffees. These may be any coffee with character, energy, and power. Kenya, with its resonant dimension and wine-like acidity, is a favorite highlight coffee among American blenders. Other blenders may prefer the round, clean power of good Costa Ricas, the twistier, often fruit-toned energy of Guatemala Antiguas, or the singing, floral sweetness of washed Ethiopias.
Bottom-Note Coffees and the Robustas Controversy. Blenders often add coffees whose primary contribution to the cup is weight and character. Bottom-note coffees are particularly important in espresso, where resonance and body are paramount. Sumatra Mandhelings are sometimes used as bottom-note coffees, as are specially handled coffees like India Monsooned Malabar and aged coffees. The most controversial bottom-note coffees are the heavy but inert coffees of the robusta species. Italian blenders use robustas freely in their espresso blends to promote body and the formation of crema, the golden froth that covers the surface of a well-made tazzina of espresso. North-American blenders use robustas in their espresso blends very sparingly, if at all. I have waffled about the value of robustas in espresso blends over the years, but at this moment I am a robusta nay-sayer. To my palate robustas are simply too dull and lifeless. They may contribute body, but in return they tend to suck life out of a blend, absorbing nuance and deadening the profile.