Blueprints are used to upgrade different parts of your car and improve your car's overall performance. Higher grade blueprints usually result in better performance.
Blueprints usually drop from winning different races (Underground, Toyo, Live Races, etc.
You can also buy blueprints for your car on the in-game market.
How to use it:
After receiving the blueprint you can find it in your Storage (Garage-Items). You will also see it once you enter the Upgrade section in your Garage.
If you got the cash and kits required to install the blueprint, go to the upgrade section of the car and part; on the bottom you should see the blueprint icon - just tap on it and then click install.