There is a fundamental difference between petrol and diesels. Petrol engines have a throttle butterfly to control power. Diesels control Torque using varying quantities of injected fuel. When you let off the accelerator pedal in a diesel, you reduce fueling, the exhaust energy reduces, and the turbine freewheels down to a low speed of its own volition. In a Petrol however, the throttle butterfly is after the turbo compressor. As soon as you let off the accelerator, the throttle plate closes, blocking off all air flow from the compressor, since the compressor is spinning hard, it will try to force air into a closed pipe. It can't, so it undergoes surge. That is bad for turbo chargers.
To stay away from that, you have blow-off valves, which divert the pressurised air back into the inlet of the compressor. So the compressor "freewheels". Now this is the important part. the blow off valve lets the compressor maintain its speed when you let of the throttle. So the next time you press down on the accelerator, you do not need to wait to get the turbo up to speed. As soon as the throttle opens, the blow-off deactivates, and you've got boost to play with again. Once a turbo is spooled up, you can play around with the accelerator position and you will not have that much lag honestly.
Also, boost control at part load is a shared work between the blow-off valve and the wastegate. Or rather, at full load when the throttle blade is open, it's the wastegate that manages boost. At part load when the throttle blade is partially open, the blow-off valve regulates the intake pressure. So yes, at part load, you will hear the blow off valve continuously open. Boot it, and the noise goes away till the next time you let off the throttle.
Besides it would be a fairly crappy engine if it keeps on giving torque even when you've let off the accelerator