The basics of blending consists of finding the lost flavors of one specific single origin bean in another bean. Therefore perfecting both (or many) beans in the same cup.
As a result, a good blend mostly has many axes. For example, the timing is important. Palate has a base, mid flavors and finishing flavors. When you taste the cup you first taste the base, then the middle flavors. Finishing flavors remain for a while. A blend tries to balance this.
Also, the balance in between sweetness, bitterness could be managed with the selected single origin beans. Acidity or body could be set to desired levels. All of these could be crafted on all axes to perfection.
The espresso blend
As a result of the preparation technique, espresso is acidic and less bodily then its counterparts. As a result, when espresso blends are prepared, less acidic and more bodily origins are preferred.
Another observable difference in espresso blends is their roast. Darker roasts makes any origin less acidic and more bodily. So, many espresso blends are roasted a bit darker than normal to achieve a better flavor.