When you block a blow off valve and it stalls what does that mean?

When a blow-off valve is blocked and the car stalls, it means that the air-to-fuel ratio in the engine has become too rich (i.e. there is not enough air mixed with the fuel). This is because the blow-off valve is designed to release excess boost pressure from the turbocharger/supercharger when the throttle body is closed, such as when the driver shifts gears. When the valve is blocked, the pressure builds up in the intake manifold and the fuel injectors continue to spray fuel into the engine, leading to a rich mixture. The rich mixture does not burn properly, causing the engine to lose power and eventually stall.