Blueprints come in many types and specifications, and each corporation has its own version of every type.
Development
Each corporation has teams of researchers working on improving the blueprint details, though these days most of the improvements are likely invisible to the blueprint users (capsuleers) and more in line with cost savings on the corporation's side, with the majority of R&D time going into running heavy tests on new schematics to ensure the resulting items can stand up to repeated use.
Under all circumstances, the blueprint must confirm to the official standard required to produce a module or ship of that particular type (with all the required attributes). Beyond that, the blueprint's details may be anything, and will vary from corporation to corporation - although, due to the strict official standards, the interiors of each item tend to be fairly similar.
Blueprints are compiled with everything from mathematical equations and subatomic particle diagrams to engineering drawings, schematics, and performance data. These can be displayed on a holoprojector, and can be stored on small data disks.
Each blueprint is different, even ones for the same items. All a blueprint does is show you a way to manufacture a given item in some way that the end result will meet the specifications set forth by the authorities. This includes not only the attributes of the item, but what amount of material you have to include for the item to be considered structurally sound.
Tech improvements
Blueprint improvement is a career unto itself and requires active training under simulated conditions. Drones, guns and armor are usually easier to work with, whereas electronics and shields are more complicated; actual ships trump everything else in complexity. From a scientific/operational point of view (as opposed to just what the capsuleers and other consumers see), the technological transition from a Tech I to a Tech II blueprint involves the application of preestablished data from datacores - extracted with the aid of data interfaces/decryptors specifically designed for that purpose - along with certain skills on the capsuleer's behalf. The data is composed of agglomerations of design patterns that skilled crews can turn into direct improvements to particular blueprints.
Lab personnel working on improving blueprints need to sign confidentiality waivers. They often log things carefully, and though they can't keep logs of blueprint contents they are able to save things like patterns that help with speeding up the process for other items of the same type.