Compressor surge on 50 trim?

[blackrosenova400]

A blow off valve works on the premise of a pressure drop. Ideally pressure on both sides of the valve should be the same until the throttle is released. In your case it may not be, and may be caused by a boost leak at the blow of valve's signal pressure, or the signal pressure hose is too small, too long, kinked, or is bulging.

Also it could be a bad signal location, near one cylinder from the intake manifold, signal should be near the throttle as close as possible.