Is there a possibility that dirt could enter the engine through the trumpets?

No. A blow off valve is a one-way device, air will only ever flow OUT of it.

The turbo piping before the throttle body will only ever be at atmospheric or positive pressure, except for a very brief period when you rapidly open the throttle from off boost conditions.

In such a situation, if the valve is open, it will immediately close as soon as the throttle is opened, because the manifold vacuum no longer holds the piston open, and any vacuum in the turbo piping will also tend to pull the valve shut. Therefore there is no way unfiltered air can ever enter the engine through the valve.