It seems a little buggy.
I made a blueprint book with some factories, and then when I tried to use them in another save the interior of the factory was the interior of a different factory, or empty, or a random bit of a different blueprint in the book.
Factory blueprints do not support being moved to different save files or with blueprint strings. It is only guaranteed to work in the save that you created the BP from.
This is because the blueprint save data is stored as a bounding box, instead of a detailed list of entities.
Blueprints in factorio have limited mod support, so this workaround was the most reliable option.