This has been discussed before on other forums; I remember a fairly good statistical analysis of it ... if I can find it, Ill post a link. In a nutshell and from memory, the maths works out that theres a heckuva lot of variation for any blend component thats less than something like 25 or 30%. The implication for blending is that keeping it simple is probably going to keep the flavour more consistent.
Another level of complexity is that different beans are of different sizes and weights; presumably there is some separation because of it in the bag and in the hopper. For example, if you make a blend of 50% maragogype (screen size = 20+) and 50% mokka (screen size <10), how will the blend turn out when you tip it into the grinder? Will the mokka tend to settle into the bottom of the bag and, so, end up in the top of the grinder when you tip it out?