Blueprints are made by Architects using the Architect Skill and the in-game Architecture Tool.
A Blueprint (a plan of the structure) is required before you place and build the structure in question.
It's ideal to learn Architecture yourself or find a friend who is looking to take it as their main Profession, as the skill level of the Architect will determine how advanced a building can be, including the total amount of walls and floors the building can cater for.
Blueprints will be physical items within the game and similar to Contracts whereby they are printed onto Parchment, can be sold or traded. This means that an Architect could be your virtual job within Elyria, and as everyone is sure to want to build their own home, will be high in demand.