The blueprint's waiting in a pretty specific spot during the happiness for everyone mission, and you'll need to make your way to the infirmary area in duga. What you're looking for is this underground section between the infirmary and military unit pois, and the real giveaway is a hole in the wall that's got a tripwire setup. Once you spot that tripwire, you're definitely in the right place. The safe containing the blueprint is tucked away through there, and its actually pretty easy to miss if you're rushing through. A good tip is to keep your eyes low when searching since the hole isn't exactly at eye level, and watch your step around that tripwire cause it's not just for show. Finding this spot before finishing the mission makes things a lot smoother, since the area can get a bit more complicated later on.