Refer to figure.
The correct labels for each part of the propeller are as follows:

• A → This is the plane of rotation. The 2D area where the blades spin, perpendicular to the axis of rotation.
• B → This is the blade chord just like for a wing, the chord line of a propeller blade is the line connecting the trailing edge and the leading edge. The angle this makes with the plane of rotation is the blade angle.
• C → This is known as the 'blade back'. Although it is at the front of the aircraft, it is the top/back surface of the blade relative to the airflow.
• D → This is the 'blade face'. Again, although it appears to be the 'back' of the blade, it is the part of the propeller blade which faces and pushes against the oncoming air.
• E → This is the 'spinner'. An aerodynamic cover which reduces drag slightly, and directs the air to the blades.

So, the letter label C is the 'blade back'. It is worth remembering that the blade face and back are relative to the oncoming air, and not the nose of the aircraft!