Blueprints aren't terribly hard to find. So far we've found 2 for an FN FAL, 2 for an HN Needler, 1 for a Rocket Launcher, and 1 for Metal Armor MK2, all between two guys, and I doubt we've hit 100 locked containers between us. They only show up in locked lockers which occur in encounters. These were all found down in the Boneyard, except the armor print which was found by Reno. My guess is that something like this depends on your build's luck, which would make sense since it was taken from the req's of most perks this session.