Blade inertia and rotor kinetic energy are NOT the same thing, but they are related.
Blade inertia is a characteristic of the blade. It is a quantity that shows how difficult it is to change the blade speed (either by acceleration or deceleration) The bigger the inertia of the blade, the harder it is to make it move or make it stop once it's moving. It has nothing to do with the speed/RPM of the blade, it has the same value regardless if the rotor is turning or not. It depends on the way the blade is built (how heavy it is and how is its weight distributed).
Rotor kinetic energy depends on inertia AND rotor speed (RPM). So, a rotor when it's stopped has inertia but has no kinetic energy.
Once you start turning the rotor you start building kinetic energy in it using power from the engine. If rotor inertia is big, the engine will work harder to accelerate the rotor BUT, if the engine fails in flight the bigger inertia will oppose RPM decay giving the pilot more time to perform the proper actions.