Even if all these had been against night heavy bombers it's a tiny part of the losses
I think you're also overestimating how easy it would be to spot a blacked out fighter at night. A popular german night fighter tactic was often to sneak up beneath a bomber and then pitch the nose of the fighter up sharply while firing, racking the bomber's belly with cannon fire.
Many fighters were equipped with guns mounted in a vertical configuration called Schräge Musik. The guns would fire automatically when a magnetometer detected an enemy bomber was overhead.
And in Wald's research a night fighter still had some difficulty finding and approaching abomber even with ground radar - so they fired at it (using proximty fuses or upward firing guns) essentially at random points - there was no statsistical bias of the fighter pilot targetting specific systems
It's difficult to believe a WWII era fighter diving into a stream of bombers, at night, while the multiple gun turrets on 100s of bombers were returning fire - were making precise tactical decision about which part of the airframe to target.
The nice thing (statistically) about AA fire is that it's rather uniformly random.