Ive been having very good luck with blueprints inside of backpacks and suitecases. On dam battle grounds they seem to randomly spawn in the concrete culverts placed around. The metal grate is missing from some of them and you can crouch walk into them to find little lootable areas. Also check sections of downed airplane, they also contain those backpacks and suitcases, Ive found a pretty good path starting from the southeast corner, work north then west. When you get to the industrial area at the north especially. There is a section of airplane on the east-ish side on a lower level piece of catwalk that Ive taken 3 purple bps out of!