In Guns, Germs, and Steel, these two terms are used mostly in Chapter 12. In this chapter, Diamond discusses writing systems. He wants to focus on why Europeans had writing and most others did not. In this discussion, he also brings up the issue of how people come to write. He says that blueprint copying and idea diffusion are two ends of a spectrum of ways in which people can learn to write from others who have already invented writing. This idea is presented on p. 224 in the paperback edition of the book.
Blueprint copying is the most direct form of copying. In this case, Society A learns that Society B has developed writing. Society A looks at Society B’s system and essentially copies it. It might modify the system a little, but it essentially is just using what Society B has already invented.
Idea diffusion is the least direct form of learning to write. In this case, Society A learns that Society B has developed writing. However, it really knows very little about the details of Society B’s system. Therefore, Society A has to invent its own writing. All it has gotten from Society B is the idea that writing is possible.