Loss or reduction of effectiveness of the rudder (loss of aerodynamic directional control) as a result of a gross disturbance of the airflow upstream of the rudder.
The difference between blanking and stalling is that blanking is the result of some upstream effect while stalling is the result of operating the surface itself at too large an angle of attack.
Rudder blanking is a particular risk to aircraft with rear mounted engines and "T" tails, where the activation of thrust reversers deflects air forcefully forwards of the engines to create reverse thrust. The exhaust from the engines, deflected in this way, disrupts the airflow upstream of the rudder.
When in reverse thrust, buckets behind some rear-mounted engines to deflect thrust forward thereby disrupting airflow upstream of the rudder
On aircraft types that are susceptible to rudder blanking, manufacturers recommend limits to the engine thrust when deploying thrust reversers, especially on contaminated runways, where directional control from nosewheel steering is least effective.
Accidents and Incidents
MD88, New York La Guardia USA, 2015: On 5 March 2015 a Boeing MD88 veered off a snow-contaminated runway 13 at New York La Guardia soon after touchdown after the experienced flight crew applied excessive reverse thrust and thus compromised directional control due to rudder blanking, a known phenomenon affecting the aircraft type. The aircraft stopped partly outside the airport perimeter with the forward fuselage over water. In addition to identifying the main cause of the accident, the Investigation found that exposure to rudder blanking risks was still widespread.
MD82, Little Rock USA, 1999: On 1 June 1999, an MD82 belonging to American Airlines, overran the end of the runway during landing. The captain and 10 passengers were killed.