I believe the discussion is being a little over simplified. There is a difference between blocking the inlet versus blocking the outlet. This can easily be seen with many common pumps/blowers laying around your house or shop. While it is correct that with a blockage there is no total airflow through the system, what will be different is the pressure differential across the pump/blower. It takes work to create a pressure differential.
When the inlet is blocked, there is no available makeup air to enter into the blower. As a result, there is very little pressure differential across the blower. What differential does exist is also from less dense air due to the resulting partial vacuum. The outlet pressure will be atmospheric, and the inlet will be slightly below that.
When the outlet is blocked, there is an abundant supply of makeup air, but no where for it to go. As a result, there is a significant pressure differential across the blower. The inlet will be at atmospheric pressure, and the outlet will be significantly higher.
You can even demonstrate this for yourself with a common vacuum cleaner if you can totally block the exhaust. (Most of them will prevent this though.) Block both inlet and outlet to get a baseline sound of the motor. Then release the inlet and make note of the tone of the motor; it should drop in pitch as the motor loads. Repeat this, but this time release the outlet. The motor tone should remain nearly unchanged.
The other thing that is being overlooked is that air is still being moved from a low to high pressure. Even if the total air flowing through the system remains zero, there is air flowing (and leaking back) through the blower.
By the way, this becomes even more pronounced on a pump, where you can nearly stall the pump with a blocked outlet, and it will run nearly free with a blocked inlet.