When we blend, we like to use a top, middle and bottom note, or at least a top and bottom.
Top notes tend to get lost in an equal ratio blend, so some of our blends use two parts top to one parts middle and bottom note.
I consider Ethiopian yirgacheffe and sidamos to be top notes along with the costa ricans we have experience with. The mexican Chiapas we are roasting "light" also makes a nice top note.
We use Guats and Costa Ricans for mid notes and sometimes bottoms, depending on the circumstance.
Sumatras are great bottom notes as well as certain offerings like Peruvians and Bolivians, (when roasted dark.)
Most of our blends contain two or three beans. (We roast and then blend.)
Our espresso blends are made up of low-acid beans roasted to different levels starting on the dark side of a medium roast. My favorite espresso blend is made up of three arabica beans and a small percentage of a high-quality robusta.