Petrol engines have a throttle that closes when you take your foot off the accelerator. When the throttle closes the air is blocked - the blow-off valve opens when the pressure rises due to the blocked flow. As the air from the turbo then flows through the valve, the turbo can keep spinning for when you put your right foot down again.
By comparison, a 300Tdi (and nearly all conventional diesels) does not have a throttle and the air from the turbo continues to flow as normal for the engine speed - only the fuel is reduced when you take your foot off the accelerator. A blow-off valve will have no affect.
Some later diesels do have a throttle - I am not familiar with any of these and suspect it must be something to do with emission/computer systems, possibly connected with air flow measurement (but I stand to be corrected).

