It's a matter of pressure. The higher the pressure, the easier to cut a surface, i.e. the skin of the apple.
Pressure is defined as force per surface-unit.
A blunt knife edge will present a greater surface area to the apple than a sharp knife would.
So the same force is distributed over a larger area, and the pressure on the apple skin is a lot lower.
A blunt knife is more dangerous to use than a sharp one, because:
(1) you use more force, with a greater risk of slipping
(2) blunt knives leave more jagged wounds.
Of course in both cases you need good and safe cutting skills.