It took me a while to locate this myself. PlanBuild saves blueprints into a BepInEx folder in the same location as your Valheim.exe file. Since I installed Valheim from Steam on Windows, my blueprints are saved here:
There won't be a blueprints tab until you create a blueprint and save it to the Blueprints Category. Edit: Other than the Tools tab, there will be a separate tab for each blueprint Category you name.
