Blocking a fan makes the blades speed up. This is easiest to observe with a vacuum cleaner: the motor increases in pitch when the nozzle gets occluded. Why does making it harder to pump the air actually make the blades easier to spin?
Fan blades, being airfoils, generally have a high lift to drag ratio. This means that they push the air perpendicular to the plane they are in. Since they are tilted slightly, "perpendicular" means the air is pushed mostly forward (out the fan) and a little prograde (i.e. they impart a little bit of swirling motion to the air). The angular momentum in this swirling motion must be replaced by the motor.
When a fan is blocked, the air starts to be pushed forward but "piles up" against the blockage. The trapped air ends up swirling around which makes it harder for the blades to impart further angular momentum to it. Thus there is less torque on the motor and it speeds up. Is this line of reasoning correct?
A similar argument can be made for blowers which throw the air out like a centrifuge: when airflow gets blocked there won't be new air passing through blades and gaining angular momentum from them.