Clarification is needed on the part. "Blowback valve" isn't a specific name for anything. This item is similar, a loading nozzle:
http://www.ascarmoury.com/product_in...roducts_id=264
Or is he referring to the rocket valve / reed valve / power bulb / etc? (this part has many different names)
http://www.wgcshop.com/WGC_Shop/imag...pt_tm226cb.jpg