Bow blueprints were attainable in the game from 26 April 2013 to 6 May 2013. Bow blueprints were dropped by a monster or won through the Squeal of Fortune. They were an "uncommon" reward in the Squeal of Fortune. Almost any monster could drop them, though higher levelled monsters, including bosses such as the Queen Black Dragon, had higher drop rates. They are still used to create starfire or starfury bows if you got them during the release dates mentioned above.
The dialogue that appears when you "Read" the Bow blueprints: The blueprints needed to create a starfire or starfury bow. I should talk to Ivar at the Artisan's Workshop for more information about this item.