Take this as the absolute answer, this is directly from many text books over here in Australia (including those from Aviation Theory Centre, Bob Tait, other various books, and even on the internet...)
It's a pity I can't draw a diagram here for you, I get this question from my students all the time.
Put simply...
The
Blade Back is the most curved part of the propellors aerofoil that you can't see when sitting in the cockpit of a single engined aircraft.
the
Blade Face is the flate side that
Faces towards you.
Now heres a real hard question about propellor theory...
What is the Experimental Pitch of a propellor?
