Blueprinting [also known as balancing] is when every rotating item in the lower end is [surprise] spin balanced to eliminate inherent vibrations. On some engines, the rotating assembly can be item balanced, where the crank is balanced seperate from everything, and then the crank pulley is balanced seperate, then the flywheel is balanced and then the pressure plate is balanced while the pistons and rod assemblies are brought to near-identical weights soas not to throw off the balance. The benefit of this process is that there is less energy [HP] wasted in creating vibration, and at the same time, there is less internal stresses generated in the crank, which leads to more reliability, less stress on the main bearings, and just a generally more sound tuning base to start from, whether you're after power or economy.