This is a very complicated question, but I will try to answer you as simple as I can.
1. Why blend?
a. In business, people blend lower quality to make more profit. As simple as that. (probably #1 reason for blending)
b. or, they want to create their own signature flavors (like "Charbucks" carbony flavor).
c. or, trying to achieve better flavor profile than SOC (Single Origin Coffee).... if you do not know what SOC means, you need to research your own.
2. Do I blend two or 3 green coffee together and roast them at the same time? or roast all 3 beans separately and blend?
that would be depends on the coffee characteristics.
as an example, if you want to blend "Maragogype" (biggest coffee beans, size of Elephant) and Mocca beans (smallest coffee bean. size of a cat), you must roast separately and blend because if you roast them together, smaller beans will be burnt before Maragogype gets brown. (photos are attached below)
or, if you want to use 6500 feet high typica beans which are very hard dense beans, with 2500 feet Prime beans which is very soft beans, you also can not roast them together. Prime beans will burn. (Tip : higher altitude beans are harder and dense beans. need more time and more heat to roast. Lower altitude beans are softer and less dense. So less heat and shorter time. Generally speaking. BTW, Denser and harder beans are better beans)
So, it is extremely important to realize each beans characteristics before you blend and roast.
2. how to choose my coffee bean blends?
Depends on what kind of mood I am in.... yes, kind of.
I work in Guatemala and our country has fine acidity & body with good balance. You can even use same beans (let's say Bourbon from Antigua). you can use 60% of Bourbon roasted full city plus. and 40% of same beans, roasted City. This creates more and deeper flavors, just by mixing two different types of roasts with same beans.
or.
Sometimes, I craved for sharper + sweeter + aromatic, then, you can add about 25% of central american beans (SHB) with other country (such as Brazil, Indonesia..etc)
But sometimes if I like, more body + really sweet, then add 35% of Sumatra. You will gain body but you will lose sharpness.
There are millions and millions of combinations..... literally..
3. what makes a great blend? which two countries...etc
Personally, I like Ethiopia(or Kenya) - these two countries sometimes share very similar characteristics & Indonesian (Sumatra or Sulawesi).
Anyway, this is very complicated subject and tough to explain everything here.
