Blending coffee can be a tricky task. Whether you are blending for coffee or blending for espresso, each has their own difficulties.
I have not been blending long enough to pinpoint all of the pertinent differences, but between the two, I find espresso blending to be a little more difficult. Which is a simple way of saying, "so much to learn..."
Sometimes I use Brazilian as a base, sometimes I don't. I try to match general origin, bean size/moisture content, or both if I preblend; otherwise, I roast each bean separately. Sometimes roasting separately takes too damn long, and sometimes the percentages are so low that its not feasible to do in small amounts.
I think about what each bean will lend to the blend, whether it is a lower toned chocolate, a higher fruited note, or maybe a middle of the road nutty flavor. I think in terms of washed, semi-washed, and dry process when thinking about crema, mouthfeel, and persistency of flavor.
What am I using as my base?
Is this bean X adding or subtracting? In what way?
What degree of roast is optimum for each bean?
Do I use beans with the same approximate optimum roast temperature?
Wait three days.
Test. Test. Test. Change brew temp. Test test test. Change brew temp. Test test test.
Finally come to a conclusion, and either praise or damn the beans.
It's never the roaster's fault... really. *grin*