A blueprint is a mental idea for a design of something that will have a function. There can be blueprints for houses, cars, various machines and so on. This is a plan which must be present in order to coordinate the construction of anything such as complicated creatures with diverse body plans. The builder can only assemble different component parts into a working system if he knows where he is going with the process.
In biology, the body plans of different organisms each follow a specific plan. The body plan of starfish and relatives calls for five or multiples of five equal parts. The body plans of four-footed creatures all include a backbone. The plan for the camera eye includes some specific components like a focusing lens. There can be blueprints also for functions like biological clocks in all living organisms. The way that the clock works is similar but the proteins carrying out the task are different, just as a builder can use a common blueprint to build one house with expensive materials and another with cheaper materials.